__i<magic>__ method incompatible with __<magic>__ of superclass

I'm recieving an a error within folowing code:

class A:
    def __add__(self, other) -> int:
        ...
    def __iadd__(self, other) -> 'A':
        ...

class C(A):
    #  error: Return type of "__iadd__" incompatible with "__add__" of supertype "A"
    def __iadd__(self, other) -> 'C':
        ...

I'm not sure that __add__ and __iadd__ methods have to be compatible.
And, if belief to the MyPy in the signature of class A, that not produce the error, MyPy also doesn't think they have to be compatible...