You were getting error because listOf(1, 2, null, 4) creates List<Int?>, and in your second line, the returning collection from .filter{ it != null} will also be typeof List<Int?>. Answered for somebody else in future 
You were getting error because listOf(1, 2, null, 4) creates List<Int?>, and in your second line, the returning collection from .filter{ it != null} will also be typeof List<Int?>. Answered for somebody else in future