Facebook redirect on "Don't Allow" uses unexpected query string: ?error[type]=OAuthAccessDeniedException

Question

I'm implementing a facebook app and use oauth 2.0 to log the user in to facebook, and to request some permissions.

If the user chooses Allow, I get the expected response on the redirect_uri (with code parameter which I can exchange to an access_token). This is all working correctly.

However, when the user chooses Don't Allow on the Facebook "Request for Permission" page, the user should be redirected to my redirect_uri with the parameter error_reason (according to the specs on http://developers.facebook.com/docs/authentication/).

The user is being redirected, but the parameters are not what I expect them to be: http://myhost/fb/postauth?error[type]=OAuthAccessDeniedException&error[message]=The+user+denied+your+request.

I can't find any reference to error[message] (PHP style parameter) or the OAuthAccessDeniedException. What's going on!?!?!