Command failed due to signal: Segmentation fault: 11 - When Overriding a method

I'm trying to Mock a class for use in my test.

func testExample() {

    class SomeClassMock : SomeClass {
       override func doSomthing() ->Bool  {
            return false
        }
    }
}

But every time I get this error:

Command failed due to signal: Segmentation fault: 11