mypy AnyStr gives Incompatible types in assignment on trivial example

given the following class:

from typing import AnyStr

class A(object):

    def __init__(self, param):
        # type: (AnyStr) -> None
        self.a = param # type: AnyStr

I get the following output:

$ mypy . -v
LOG:  Mypy version 0.521
LOG:  Build finished in 1.199 seconds with 10 modules, 2076 types, and 2 errors
test.py:8: error: Incompatible types in assignment (expression has type "str", variable has type "AnyStr")
test.py:8: error: Incompatible types in assignment (expression has type "bytes", variable has type "AnyStr"

How come this assign operation gives incompatible type?