Coroutine/async il2cpp exception

Have weird exception when try to build project with my .dll:> IL2CPP error for type 'Namespace.Subnamespace.MyClass/d18' in assembly '/Folder/MyLib.dll' Additional information: Build a development build for more information. Interface System.Runtime.CompilerServices.IAsyncStateMachine method System.Void System.Runtime.CompilerServices.IAsyncStateMachine::MoveNext() not implemented on non-abstract class Namespace.Subnamespace.MyClass/d18> Everything works in Editor. Error happens even if MyFunk is empty.