Async Entity Framework Core Query Throws InvalidOperation Exception

I'm doing a complex async database query with Entity Framework Core which looks like this:

Schedule schedule = await db.Schedules
    .Include("Timetable.Class.Lecturers")
    .Include("Timetable.Class.AdvancedClassSections.ClassSessions.Lecturers")
    .Include("Timetable.Class.ClassSessions.Lecturers")
    .Include("Timetable.Class.AdvancedClassSections.Lecturers")
    .FirstOrDefaultAsync(k => k.ScheduleId == scheduleId);

But I keep getting a runtime error message which says:

System.InvalidOperationException: 'The model item passed into the ViewDataDictionary is of type 'System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1+AsyncStateMachineBox`1[System.Collections.Generic.List`1[Emily.Models.Timetable],Emily.Controllers.TimetableController+d__21]', but this ViewDataDictionary instance requires a model item of type 'System.Collections.Generic.IEnumerable`1[Emily.Models.Timetable]'.'

It seems there's a low-level operation which should cast from IEnumerable but is getting something more complex. It was working fine when it wasn't async. I have other queries like that which are running fine, but I can't seem to figure why this one throws an exception. It has nothing to do a view because this happens in a controller action.

Full Call Stack:

Microsoft.AspNetCore.Mvc.ViewFeatures.dll!Microsoft.AspNetCore.Mvc.ViewFeatures.ViewDataDictionary.EnsureCompatible(object value) Line 212 C# Microsoft.AspNetCore.Mvc.ViewFeatures.dll!Microsoft.AspNetCore.Mvc.ViewFeatures.ViewDataDictionary.ViewDataDictionary(Microsoft.AspNetCore.Mvc.ViewFeatures.ViewDataDictionary source, object model, System.Type declaredModelType) Line 138 C# [Lightweight Function]
Microsoft.AspNetCore.Mvc.Razor.dll!Microsoft.AspNetCore.Mvc.Razor.RazorPagePropertyActivator.Activate(object page, Microsoft.AspNetCore.Mvc.Rendering.ViewContext context) Unknown Microsoft.AspNetCore.Mvc.Razor.dll!Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderPageCoreAsync(Microsoft.AspNetCore.Mvc.Razor.IRazorPage page, Microsoft.AspNetCore.Mvc.Rendering.ViewContext context) Unknown Microsoft.AspNetCore.Mvc.Razor.dll!Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderPageAsync(Microsoft.AspNetCore.Mvc.Razor.IRazorPage page, Microsoft.AspNetCore.Mvc.Rendering.ViewContext context, bool invokeViewStarts) Unknown Microsoft.AspNetCore.Mvc.Razor.dll!Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderAsync(Microsoft.AspNetCore.Mvc.Rendering.ViewContext context) Unknown Microsoft.AspNetCore.Mvc.ViewFeatures.dll!Microsoft.AspNetCore.Mvc.ViewFeatures.ViewExecutor.ExecuteAsync(Microsoft.AspNetCore.Mvc.Rendering.ViewContext viewContext, string contentType, int? statusCode) Line 128 C# Microsoft.AspNetCore.Mvc.ViewFeatures.dll!Microsoft.AspNetCore.Mvc.ViewFeatures.ViewExecutor.ExecuteAsync(Microsoft.AspNetCore.Mvc.ActionContext actionContext, Microsoft.AspNetCore.Mvc.ViewEngines.IView view, Microsoft.AspNetCore.Mvc.ViewFeatures.ViewDataDictionary viewData, Microsoft.AspNetCore.Mvc.ViewFeatures.ITempDataDictionary tempData, string contentType, int? statusCode) Line 104 C# Microsoft.AspNetCore.Mvc.ViewFeatures.dll!Microsoft.AspNetCore.Mvc.ViewFeatures.ViewResultExecutor.ExecuteAsync(Microsoft.AspNetCore.Mvc.ActionContext context, Microsoft.AspNetCore.Mvc.ViewResult result) C# Microsoft.AspNetCore.Mvc.ViewFeatures.dll!Microsoft.AspNetCore.Mvc.ViewResult.ExecuteResultAsync(Microsoft.AspNetCore.Mvc.ActionContext context) Line 33 C# Microsoft.AspNetCore.Mvc.Core.dll!Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.ResultNext<Microsoft.AspNetCore.Mvc.Filters.IResultFilter, Microsoft.AspNetCore.Mvc.Filters.IAsyncResultFilter>(ref Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.State next, ref Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Scope scope, ref object state, ref bool isCompleted) Line 1338 C# Microsoft.AspNetCore.Mvc.Core.dll!Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeNextResultFilterAsync<Microsoft.AspNetCore.Mvc.Filters.IResultFilter, Microsoft.AspNetCore.Mvc.Filters.IAsyncResultFilter>() Line 1354 C# Microsoft.AspNetCore.Mvc.Core.dll!Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.ResultNext<Microsoft.AspNetCore.Mvc.Filters.IResultFilter, Microsoft.AspNetCore.Mvc.Filters.IAsyncResultFilter>(ref Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.State next, ref Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Scope scope, ref object state, ref bool isCompleted) Line 1258 C# Microsoft.AspNetCore.Mvc.Core.dll!Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeResultFilters() Line 1090 C# Microsoft.AspNetCore.Mvc.Core.dll!Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Next(ref Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.State next, ref Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Scope scope, ref object state, ref bool isCompleted) Line 853 C# Microsoft.AspNetCore.Mvc.Core.dll!Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeNextResourceFilter() Line 952 C# Microsoft.AspNetCore.Mvc.Core.dll!Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Next(ref Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.State next, ref Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Scope scope, ref object state, ref bool isCompleted) Line 557 C# Microsoft.AspNetCore.Mvc.Core.dll!Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeFilterPipelineAsync() Line 232 C# Microsoft.AspNetCore.Mvc.Core.dll!Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeAsync() Line 70 C# Microsoft.AspNetCore.Routing.dll!Microsoft.AspNetCore.Routing.EndpointMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext httpContext) Line 87 C# Microsoft.AspNetCore.Session.dll!Microsoft.AspNetCore.Session.SessionMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext context) Line 123 C# Microsoft.AspNetCore.Authorization.Policy.dll!Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext context) Line 75 C# Microsoft.AspNetCore.StaticFiles.dll!Microsoft.AspNetCore.StaticFiles.StaticFileMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext context) Line 75 C# Microsoft.AspNetCore.StaticFiles.dll!Microsoft.AspNetCore.StaticFiles.StaticFileMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext context) Line 75 C# Microsoft.AspNetCore.HttpsPolicy.dll!Microsoft.AspNetCore.HttpsPolicy.HttpsRedirectionMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext context) Line 74 C# Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore.dll!Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore.MigrationsEndPointMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext context) Line 50 C# Microsoft.AspNetCore.Localization.dll!Microsoft.AspNetCore.Localization.RequestLocalizationMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext context) Line 95 C# Microsoft.AspNetCore.Authentication.dll!Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext context) Line 65 C# Microsoft.AspNetCore.Diagnostics.dll!Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddlewareImpl.Invoke(Microsoft.AspNetCore.Http.HttpContext context) Line 80 C# Microsoft.AspNetCore.HostFiltering.dll!Microsoft.AspNetCore.HostFiltering.HostFilteringMiddleware.Invoke(Microsoft.AspNetCore.Http.HttpContext context) Line 51 C# Microsoft.WebTools.BrowserLink.Net.dll!Microsoft.WebTools.BrowserLink.Net.BrowserLinkMiddleware.InvokeAsync(Microsoft.AspNetCore.Http.HttpContext context) Line 53 C# [Resuming Async Method]
System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.Threading.Tasks.VoidTaskResult>.AsyncStateMachineBox<Microsoft.WebTools.BrowserLink.Net.BrowserLinkMiddleware.d__6>.ExecutionContextCallback(object s) Line 287 C# System.Private.CoreLib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state) Line 183 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.Threading.Tasks.VoidTaskResult>.AsyncStateMachineBox<Microsoft.WebTools.BrowserLink.Net.BrowserLinkMiddleware.d__6>.MoveNext(System.Threading.Thread threadPoolThread) Line 324 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.Threading.Tasks.VoidTaskResult>.AsyncStateMachineBox<Microsoft.WebTools.BrowserLink.Net.BrowserLinkMiddleware.d__6>.MoveNext() Line 302 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.TaskAwaiter.OutputWaitEtwEvents.AnonymousMethod__12_0(System.Action innerContinuation, System.Threading.Tasks.Task innerTask) Line 301 C# System.Private.CoreLib.dll!System.Threading.Tasks.AwaitTaskContinuation.RunOrScheduleAction(System.Action action, bool allowInlining) Line 743 C# System.Private.CoreLib.dll!System.Threading.Tasks.Task.RunContinuations(object continuationObject) Line 3417 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.__Canon>.SetExistingTaskResult(System.Threading.Tasks.Task<System.__Canon> task, System.__Canon result) Line 446 C# [Completed] Microsoft.WebTools.BrowserLink.Net.dll!Microsoft.WebTools.BrowserLink.Net.BrowserLinkMiddleware.GetScriptTag(string requestId, bool isHttps, string hostUrl) Line 83 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.__Canon>.AsyncStateMachineBox<Microsoft.WebTools.BrowserLink.Net.BrowserLinkMiddleware.d__7>.ExecutionContextCallback(object s) Line 287 C# System.Private.CoreLib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state) Line 183 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AsyncStateMachineBox<Microsoft.WebTools.BrowserLink.Net.BrowserLinkMiddleware.d__7>.MoveNext(System.Threading.Thread threadPoolThread) Line 324 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.__Canon>.AsyncStateMachineBox<Microsoft.WebTools.BrowserLink.Net.BrowserLinkMiddleware.d__7>.MoveNext() Line 302 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.TaskAwaiter.OutputWaitEtwEvents.AnonymousMethod__12_0(System.Action innerContinuation, System.Threading.Tasks.Task innerTask) Line 301 C# System.Private.CoreLib.dll!System.Threading.Tasks.AwaitTaskContinuation.RunOrScheduleAction(System.Action action, bool allowInlining) Line 743 C# System.Private.CoreLib.dll!System.Threading.Tasks.Task.RunContinuations(object continuationObject) Line 3417 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.__Canon>.SetExistingTaskResult(System.Threading.Tasks.Task<System.__Canon> task, System.__Canon result) Line 446 C# [Completed] System.Net.Http.dll!System.Net.Http.HttpClient.SendAsync.__Core|83_0(System.Net.Http.HttpRequestMessage request, System.Net.Http.HttpCompletionOption completionOption, System.Threading.CancellationTokenSource cts, bool disposeCts, System.Threading.CancellationTokenSource pendingRequestsCts, System.Threading.CancellationToken originalCancellationToken) Unknown System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.__Canon>.AsyncStateMachineBox<System.Net.Http.HttpClient.<g__Core|83_0>d>.ExecutionContextCallback(object s) Line 287 C# System.Private.CoreLib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state) Line 183 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.Net.Http.HttpResponseMessage>.AsyncStateMachineBox<System.Net.Http.HttpClient.<g__Core|83_0>d>.MoveNext(System.Threading.Thread threadPoolThread) Line 324 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.__Canon>.AsyncStateMachineBox<System.Net.Http.HttpClient.<g__Core|83_0>d>.MoveNext() Line 302 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.TaskAwaiter.OutputWaitEtwEvents.AnonymousMethod__12_0(System.Action innerContinuation, System.Threading.Tasks.Task innerTask) Line 301 C# System.Private.CoreLib.dll!System.Threading.Tasks.AwaitTaskContinuation.RunOrScheduleAction(System.Action action, bool allowInlining) Line 743 C# System.Private.CoreLib.dll!System.Threading.Tasks.Task.RunContinuations(object continuationObject) Line 3417 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.__Canon>.SetExistingTaskResult(System.Threading.Tasks.Task<System.__Canon> task, System.__Canon result) Line 446 C# [Completed] System.Net.Http.dll!System.Net.Http.RedirectHandler.SendAsync(System.Net.Http.HttpRequestMessage request, bool async, System.Threading.CancellationToken cancellationToken) Unknown System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.__Canon>.AsyncStateMachineBox<System.Net.Http.RedirectHandler.d__4>.ExecutionContextCallback(object s) Line 287 C# System.Private.CoreLib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state) Line 183 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.Net.Http.HttpResponseMessage>.AsyncStateMachineBox<System.Net.Http.RedirectHandler.d__4>.MoveNext(System.Threading.Thread threadPoolThread) Line 324 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.__Canon>.AsyncStateMachineBox<System.Net.Http.RedirectHandler.d__4>.MoveNext() Line 302 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.TaskAwaiter.OutputWaitEtwEvents.AnonymousMethod__12_0(System.Action innerContinuation, System.Threading.Tasks.Task innerTask) Line 301 C# System.Private.CoreLib.dll!System.Threading.Tasks.AwaitTaskContinuation.RunOrScheduleAction(System.Action action, bool allowInlining) Line 743 C# System.Private.CoreLib.dll!System.Threading.Tasks.Task.RunContinuations(object continuationObject) Line 3417 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.__Canon>.SetExistingTaskResult(System.Threading.Tasks.Task<System.__Canon> task, System.__Canon result) Line 446 C# [Completed] System.Net.Http.dll!System.Net.Http.DiagnosticsHandler.SendAsyncCore(System.Net.Http.HttpRequestMessage request, bool async, System.Threading.CancellationToken cancellationToken) Unknown System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.__Canon>.AsyncStateMachineBox<System.Net.Http.DiagnosticsHandler.d__10>.ExecutionContextCallback(object s) Line 287 C# System.Private.CoreLib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state) Line 183 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.Net.Http.HttpResponseMessage>.AsyncStateMachineBox<System.Net.Http.DiagnosticsHandler.d__10>.MoveNext(System.Threading.Thread threadPoolThread) Line 324 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.__Canon>.AsyncStateMachineBox<System.Net.Http.DiagnosticsHandler.d__10>.MoveNext() Line 302 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.TaskAwaiter.OutputWaitEtwEvents.AnonymousMethod__12_0(System.Action innerContinuation, System.Threading.Tasks.Task innerTask) Line 301 C# System.Private.CoreLib.dll!System.Threading.Tasks.AwaitTaskContinuation.RunOrScheduleAction(System.Action action, bool allowInlining) Line 743 C# System.Private.CoreLib.dll!System.Threading.Tasks.Task.RunContinuations(object continuationObject) Line 3417 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.__Canon>.SetExistingTaskResult(System.Threading.Tasks.Task<System.__Canon> task, System.__Canon result) Line 446 C# [Completed] System.Net.Http.dll!System.Net.Http.HttpConnectionPool.SendWithVersionDetectionAndRetryAsync(System.Net.Http.HttpRequestMessage request, bool async, bool doRequestAuth, System.Threading.CancellationToken cancellationToken) Unknown System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.__Canon>.AsyncStateMachineBox<System.Net.Http.HttpConnectionPool.d__86>.ExecutionContextCallback(object s) Line 287 C# System.Private.CoreLib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state) Line 183 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.Net.Http.HttpResponseMessage>.AsyncStateMachineBox<System.Net.Http.HttpConnectionPool.d__86>.MoveNext(System.Threading.Thread threadPoolThread) Line 324 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.__Canon>.AsyncStateMachineBox<System.Net.Http.HttpConnectionPool.d__86>.MoveNext() Line 302 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.TaskAwaiter.OutputWaitEtwEvents.AnonymousMethod__12_0(System.Action innerContinuation, System.Threading.Tasks.Task innerTask) Line 301 C# System.Private.CoreLib.dll!System.Threading.Tasks.AwaitTaskContinuation.RunOrScheduleAction(System.Action action, bool allowInlining) Line 743 C# System.Private.CoreLib.dll!System.Threading.Tasks.Task.RunContinuations(object continuationObject) Line 3417 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.__Canon>.SetExistingTaskResult(System.Threading.Tasks.Task<System.__Canon> task, System.__Canon result) Line 446 C# [Completed] System.Net.Http.dll!System.Net.Http.HttpConnection.SendAsyncCore(System.Net.Http.HttpRequestMessage request, bool async, System.Threading.CancellationToken cancellationToken) Unknown System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.__Canon>.AsyncStateMachineBox<System.Net.Http.HttpConnection.d__61>.ExecutionContextCallback(object s) Line 287 C# System.Private.CoreLib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state) Line 183 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.Net.Http.HttpResponseMessage>.AsyncStateMachineBox<System.Net.Http.HttpConnection.d__61>.MoveNext(System.Threading.Thread threadPoolThread) Line 324 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder<System.__Canon>.AsyncStateMachineBox<System.Net.Http.HttpConnection.d__61>.MoveNext() Line 302 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.TaskAwaiter.OutputWaitEtwEvents.AnonymousMethod__12_0(System.Action innerContinuation, System.Threading.Tasks.Task innerTask) Line 301 C# System.Private.CoreLib.dll!System.Threading.Tasks.AwaitTaskContinuation.RunOrScheduleAction(System.Action action, bool allowInlining) Line 743 C# System.Private.CoreLib.dll!System.Threading.Tasks.Task.RunContinuations(object continuationObject) Line 3417 C# System.Private.CoreLib.dll!System.Threading.Tasks.Task.TrySetResult(int result) Line 401 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder.SetExistingTaskResult(System.Threading.Tasks.Task task, int result) Line 446 C# [Completed] System.Net.Http.dll!System.Net.Http.HttpConnection.CheckUsabilityOnScavenge.__ReadAheadWithZeroByteReadAsync|44_0() Unknown System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AsyncStateMachineBox<System.Net.Http.HttpConnection.<g__ReadAheadWithZeroByteReadAsync|44_0>d>.ExecutionContextCallback(object s) Line 287 C# System.Private.CoreLib.dll!System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext executionContext, System.Threading.ContextCallback callback, object state) Line 183 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AsyncStateMachineBox<System.Net.Http.HttpConnection.<g__ReadAheadWithZeroByteReadAsync|44_0>d>.MoveNext(System.Threading.Thread threadPoolThread) Line 324 C# System.Private.CoreLib.dll!System.Runtime.CompilerServices.AsyncTaskMethodBuilder.AsyncStateMachineBox<System.Net.Http.HttpConnection.<g__ReadAheadWithZeroByteReadAsync|44_0>d>.MoveNext() Line 302 C# System.Net.Sockets.dll!System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.InvokeContinuation(System.Action continuation, object state, bool forceAsync, bool requiresExecutionContextFlow) Unknown System.Net.Sockets.dll!System.Net.Sockets.Socket.AwaitableSocketAsyncEventArgs.OnCompleted(System.Net.Sockets.SocketAsyncEventArgs _) Unknown System.Net.Sockets.dll!System.Net.Sockets.SocketAsyncEventArgs..cctor.AnonymousMethod__176_0(uint errorCode, uint numBytes, System.Threading.NativeOverlapped* nativeOverlapped) Unknown System.Private.CoreLib.dll!System.Threading.ThreadPoolTypedWorkItemQueue<System.Threading.PortableThreadPool.IOCompletionPoller.Event, System.Threading.PortableThreadPool.IOCompletionPoller.Callback>.System.Threading.IThreadPoolWorkItem.Execute() Line 1143 C# System.Private.CoreLib.dll!System.Threading.ThreadPoolWorkQueue.Dispatch() Line 924 C# System.Private.CoreLib.dll!System.Threading.PortableThreadPool.WorkerThread.WorkerThreadStart() Line 121 C# [Async Call Stack] [Async] Microsoft.AspNetCore.Watch.BrowserRefresh.dll!Microsoft.AspNetCore.Watch.BrowserRefresh.BrowserRefreshMiddleware.InvokeAsync(Microsoft.AspNetCore.Http.HttpContext context) Unknown [Async] Microsoft.AspNetCore.Server.Kestrel.Core.dll!Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.ProcessRequests<Microsoft.AspNetCore.Hosting.HostingApplication.Context>(Microsoft.AspNetCore.Hosting.Server.IHttpApplication<Microsoft.AspNetCore.Hosting.HostingApplication.Context> application) Unknown [Async] Microsoft.AspNetCore.Server.Kestrel.Core.dll!Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.ProcessRequestsAsync<Microsoft.AspNetCore.Hosting.HostingApplication.Context>(Microsoft.AspNetCore.Hosting.Server.IHttpApplication<Microsoft.AspNetCore.Hosting.HostingApplication.Context> application) Unknown [Async] Microsoft.AspNetCore.Server.Kestrel.Core.dll!Microsoft.AspNetCore.Server.Kestrel.Core.Internal.HttpConnection.ProcessRequestsAsync<Microsoft.AspNetCore.Hosting.HostingApplication.Context>(Microsoft.AspNetCore.Hosting.Server.IHttpApplication<Microsoft.AspNetCore.Hosting.HostingApplication.Context> httpApplication) Unknown [Async] Microsoft.AspNetCore.Server.Kestrel.Core.dll!Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Infrastructure.KestrelConnection<Microsoft.AspNetCore.Connections.ConnectionContext>.ExecuteAsync() Unknown

Solution

Most likely when you changed this method to be async you neglected to change the code that calls to the method to await it before sending the result to the View. Since the return type of the method is now a Task<List<Timetable>> instead of a List<Timetable>, the view recognizes that it isn't an IEnumerable<Timetable> and throws an exception.

If the exception is appearing to come from this line of code, it may be due to the asynchronous task. Your method has already returned the Task, and the calling method has already invoked the View before this line of code actually finishes executing. But the stack trace doesn't appear to refer to this line of code anywhere: it clearly shows the Razor view throwing this exception when it validates the model type.