mypy error - incompatible type despite using 'Union'

Consider following code sample:

from typing import Dict, Union

def count_chars(string) -> Dict[str, Union[str, bool, int]]:
    result = {}  # type: Dict[str, Union[str, bool, int]]

    if isinstance(string, str) is False:
        result["success"] = False
        result["message"] = "Inavlid argument"
    else:
        result["success"] = True
        result["result"] = len(string)
    return result

def get_square(integer: int) -> int:
    return integer * integer

def validate_str(string: str) -> bool:
    check_count = count_chars(string)
    if check_count["success"] is False:
        print(check_count["message"])
        return False
    str_len_square = get_square(check_count["result"])
    return bool(str_len_square > 42)

result = validate_str("Lorem ipsum")

When running mypy against this code, following error is returned:

error: Argument 1 to "get_square" has incompatible type "Union[str, bool, int]"; expected "int"

and I'm not sure how I could avoid this error without using Dict[str, Any] as returned type in the first function or installing 'TypedDict' mypy extension. Is mypy actually 'right', any my code isn't type safe or is this should be considered as mypy bug?