Unicode-objects must be encoded before hashing when requesting data using Flask-OAuth

I'm integrating Google's login with a Flask site using Flask-OAuth.

Everything is working fine. I can authorise the login and get a token back etc without any difficulties. But when I use Flask-OAuth's get method to request the logged in user's email address I get an error saying:

TypeError: Unicode-objects must be encoded before hashing

I'm using Python3 and this has the smell of a Python version issue but I can't figure out what I'd need to change.

The code I'm using is this:

def get_additional_data(self):
    access_token = session.get('oauth_token')
    headers = {'Authorization': 'OAuth ' + access_token[0]}
    return self.service.get(
        'https://www.googleapis.com/oauth2/v1/userinfo', None,
        headers=headers)

I'm not sure what I can encode in that request. Even if I don't pass the headers I get the same error (rather than an invalid request or something like that).

I've run 2to3 on oauth2/__init__.py and the tweaks is suggests are very minor and shouldn't prevent the code from running in Python 3. Also, everything else OAuth2 related is working.

The bad news is that the solution to this problem is switching to Flask-OAuthlib.

The good news is it required very few changes from Flask-OAuth to get it working.