There is a different behaviour when mocking the same method using the following two Arrange overrides: public static FuncExpectation<TResult> Arrange<TResult>(Expression<Func<TResult>> expression); public static FuncExpectation<TResult> Arrange<T, TResult>(T obj, Func<T, TResult> func); Repro project attached. Steps to reproduce: 1. Open attached solution 2. In Tests.cs, run JustMockArrangeQueryableTest - it passes 3. Run JustMockArrangeQueryableTest2 - it fails with a invalid cast exception on the Residents collection.