The following runs properly: ``` def foo(a: bytes, b:bytes) -> bytes: return b'%s:%s' % (a, b) assert foo(b'a', b'b') == b'a:b' ``` However mypy complains: ``` example.py: note: In function "foo": example.py:2: error: Incompatible return value type (got "str", expected "bytes") ```