oData expand - expanding null object showing all null properties

We have an oData rest api for querying data. ( written in c# )

This is a question about how $expand works when you are expanding a null reference.

I'm not sure what the expected behavior should be.

If we do an expand with a select, any null objects are simply returned as null.

e.g.

...&$expand=HighestAlarmType ($select = ObjectID, UniversalID)&...

Gives output:

    {
        ...
        "HighestAlarmType": null
    },

But if we just do an expand without adding any columns in the select we get a "fake" expanded object with all null values showing up.

...&$expand=HighestAlarmType&...

Gives output:

        "AssetNumber": "214",
        "HighestAlarmType": {
            "@odata.etag": "W/\"\"",
            "Updates": null,
            "ObjectID": null,
            "DateAndTimeCreated": null,
            "UniversalID": null,
            "LastUpdatedOn": null,
            "LastUpdatedBy": null,
            "UserCreatedBy": null,
            "AlarmTypeName": null,
            "SeverityRanking": null,
            "RangeColorToUseOnGauges": null,
            "AlarmLevel": null,
            "FailurePFIntervalOption": null,
            "CalculateCompletionBasedonPF": null,
            "LightColorToUseOnMobile": null,
            "DarkColorToUseOnMobile": null,
            "CollectorsNoteMandatory": null,
            "DarkTextColorToUseOnMobile": null,
            "LightTextColorToUseOnMobile": null,
            "AcknowledgmentAllowFixed": null,
            "AcknowledgmentAllowIncoAlarm": null,
            "AcknowledgmentAllowIncoReading": null,
            "AcknowledgmentAllowMonitoring": null,
            "AcknowledgmentAllowWorkOrders": null,
            "AcknowledgmentAllowWorkRequest": null,
            "AcknowledgmentAllowWFromScratc": null,
            "AcknowledgmentControlMethodsBy": null,
            "PFIntervalAdjustment": null
        }

Wondering if this is expected behavior or if there is something we are doing wrong.

Solution

I think oData expand should return just "null" if the reference is null.

An example is in the northwind query:

https://services.odata.org/V4/Northwind/Northwind.svc/Employees?$filter=FirstName eq 'Andrew'&$expand=Employee1&$top=5&$count=true

This yields:

"Employee1": null

I have also looked more deeply into our code and can see we have a bug in it and should be just returning null.