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.

The problem was in Project name. Solution is

1. Right mouse click on Project name (not solution name) in Solution Explorer.
2. Properties
3. "Application" tab
4. Assembly name -> set to the same to .dll file name (without ".dll")