Test method XyzUnitTest.TestSomething threw exception: System.Runtime.Remoting.RemotingException: ByRef value type parameter cannot be null.

It turns out that this is caused by not having a return value in the expectation when returning a primitive:

The line of tested code that was failing:

int id = this.Foo.GetId(arg1, arg2, arg3);

The expectation that was causing the failure in the test code was:
Expect.AtMost(1).On(foo).Method(“GetId”).WithAnyArguments();

Need to be changed to:
Expect.AtMost(1).On(foo).Method(“GetId”).WithAnyArguments().Will(Return.Value(Id));

I think this is especially important as a (non nullable) int is being returned so the proxy is treating it as byref, hence it can not be null. This may have been missed if it was returning a object as a null object can be returned in this scenario.