FGraph::OAuthError in SessionController#oauth_callback

When calling FGraph::oauth_access_token as given by the examples, the call results into an Exception saying "(OAuthException) Error validating verification code."

As the rails webserver console says, the error is the result of the following Exception:

--- begin ---
MultiJson::DecodeError (706: unexpected token at 'access_token=XXXXXREMOVEDXXXXXX'):
app/controllers/session_controller.rb:23:in `oauth_callback'

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--- end ---