NullReferenceException when configuring expectation

[kentcb]

When configuring expectations for Akavache's IBlobCache, I did something like this:

this
    .When(x => x.GetCreatedAt(It.IsAny<IEnumerable<string>>()))
    .Return(Observable.Return<Dictionary<string, DateTimeOffset?>>(null));

But this resulted in an exception:

System.NullReferenceException: Object reference not set to an instance of an object.
   at PCLMock.MockBase`1.GetContinuationKey(LambdaExpression selector)
   at PCLMock.MockBase`1.EnsureWhenContinuationCollection(LambdaExpression memberSelector)
   at PCLMock.MockBase`1.AddOrReplaceWhenContinuation(LambdaExpression memberSelector, WhenContinuation continuation)
   at PCLMock.MockBase`1.When[TMember](Expression`1 selector)

[kentcb]

Ooops, that's because they're extension methods. All the same, it might be nice to report a nice error when this mistake is made.