I think we should show an exception here instead of performing an assert. The exception message that you are going to be getting from the assert will not help the user at all. The message you will see will be this one https://github.com/NancyFx/Nancy/blob/master/src/Nancy.Testing/Asserts.cs#L134 and it will make people scratch their heads because it makes no sense in the context of ShouldContain