Return type of send method

[dinauu]

Why does this send method have to return List[Tuple[Callable, Optional[str]]]? I have a receiver that returns List[Tuple[Callable, Dict[str, Any]]] and I get this error:

Incompatible types in assignment (expression has type "List[Tuple[Callable[..., Any], Optional[str]]]", variable has type "List[Tuple[Callable[..., Any], Dict[Any, Any]]]")

[pokidovea]

A receiver can return any type, сan't it? But why there is an Optional[str] in the signature of method .send?