The message is incorrect for the input to the function, and confusing because the caller did supply a bytes-like object.

The exception is from an implementation detail, and if the implementation changes the exception could as well. 

I suggest the implementation is changed to follow the example of  urllib.parse.unquote_to_bytes, which encodes its string parameter to bytes if it receives a str object.

Alternatively, a check for required type should be implemented and raise a TypeError with the correct error-message.