NSubstitute - TestFixture 1 causes AmbiguousArgumentsException in TestFixture 2

I am writing C# unit tests using NUnit and NSubstitute. I am testing a class which will attempt to retrieve objects from a config provider implementing the following interface:

public interface IConfigProvider<T> {
    T GetConfig(int id);
    T GetConfig(string id);
}

The class being tested only uses the int version of GetConfig so in the SetUpFixture I do the following to set up a mocked config provider that will always return the same dummy object:

IConfigProvider<ConfigType> configProvider = Substitute.For<IConfigProvider<ConfigType>>();
configProvider.GetConfig(Arg.Any<int>()).Returns<ConfigType>(new ConfigType(/* args */);

This runs absolutely fine if that TestFixture is the only one being run. However, in a different TestFixture in the same assembly, I check for received calls like this:

connection.Received(1).SetCallbacks(Arg.Any<Action<Message>>(), Arg.Any<Action<long>>(), Arg.Any<Action<long, Exception>>());

If these Received tests run before the config provider tests, then the config tests fail in the SetUpFixture with an AmbiguousArgumentsException:

Here.Be.Namespace.ProfileManagerTests+Setup (TestFixtureSetUp):
SetUp : NSubstitute.Exceptions.AmbiguousArgumentsException : Cannot determine argument specifications to use.
Please use specifications for all arguments of the same type.
at NSubstitute.Core.Arguments.NonParamsArgumentSpecificationFactory.Create(Object argument, IParameterInfo parameterInfo, ISuppliedArgumentSpecifications suppliedArgumentSpecifications)
at System.Linq.Enumerable.<SelectIterator>d__7`2.MoveNext()
at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
at NSubstitute.Core.Arguments.MixedArgumentSpecificationsFactory.Create(IList`1 argumentSpecs, Object[] arguments, IParameterInfo[] parameterInfos)
at NSubstitute.Core.Arguments.ArgumentSpecificationsFactory.Create(IList`1 argumentSpecs, Object[] arguments, IParameterInfo[] parameterInfos, MatchArgs matchArgs)
at NSubstitute.Core.CallSpecificationFactory.CreateFrom(ICall call, MatchArgs matchArgs)
at NSubstitute.Routing.Handlers.RecordCallSpecificationHandler.Handle(ICall call)
at System.Linq.Enumerable.WhereSelectArrayIterator`2.MoveNext()
at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source, Func`2 predicate)
at NSubstitute.Routing.Route.Handle(ICall call)
at NSubstitute.Proxies.CastleDynamicProxy.CastleForwardingInterceptor.Intercept(IInvocation invocation)
at Castle.DynamicProxy.AbstractInvocation.Proceed()
at Castle.Proxies.IConfigProvider`1Proxy.GetConfig(Int32 id)
at Here.Be.Namespace.ProfileManagerTests.Setup.DoSetup()

What's really confusing me is that I can observe this effect even between test runs - if I use the NUnit GUI to run the Received tests alone, and then run the config tests alone, the config tests will fail. If I then immediately run the config tests again, they will pass.

Things I've tried:

Adding configProvider.GetConfig(Arg.Any<string>()).Returns... as well, in case the overloading was the problem.
I've read the NSubstitute docs on argument matching, but I can't find a solution there. If it is a case of having to supply argument matchers for both the int and string versions of the method, I can't work out how to do that.

As it happens, the tests I'm using will only ever call the GetConfig method with values of 0 or 1, so I can just provide Returns specifications for those two values and not use matching at all, but I want to understand how to fix this more generally.

Solution

Ambiguous arguments is when NSubstitute compares the arguments to the call it is currently working with, to the stack of "argument matchers" it has (each time Arg.Blah is called, an the argument matcher is added to that stack), and it is unable to resolve which argument goes where.

Normally this is caused by having a call like blah(null, null), with a single argument matcher queued up, but it can also be caused by the stack getting out-of-sync due to an arg matcher being used outside of call configuration, or as an argument to a non-virtual method.

Version 1.8.0 (released after your question) includes slightly improved detection of the latter case, so that may be worth trying.

Other than that, I've had this problem a few times and have used the following (painful) approach.

run the test in isolation and ensure it passes
work out what test runs immediately proceeding (can usually guess, but test logs can help here), and run just those two tests. Confirm it fails.
Look for any calls to Arg.xyz that could queue up an argument matcher in either test. Make sure it is used as part of a call configuration. Sometimes working out which call is problematic can be done by commenting out lines or replacing arg matchers with other values.
Make sure there are no calls to non-virtual methods that are confusing NSubstitute.

Sometimes the problem may be due to a previous fixture, so you may need to workout the previous fixture and explore there as well. :(