Why is my Facebook application with error 104 ("invalid signature")?

I am trying to develop a Facebook application using PyFacebook (hosted on Google App Engine). It's an FBML application (runs in a Facebook canvas instead of an iframe). I'm having problems getting any API calls to function. The sequence looks like this:

fb = facebook.Faceboook(api_key, secret_key)
fb.session_key = cherrypy.reques开发者_StackOverflowt.params['fb_sig_session_key']
fb.uid =cherrypy.request.params['fb_sig_user']

Then if I try, for example:

user = fb.users.getLoggedInUser()

This will fail with:

Error 104: Incorrect signature

I'm not sure where I'm going wrong. I've verified that the api_key and secret_key are correct (many, many times). The application has been added to my account. I've even been able to get a desktop application working; my problem is only with this web-hosted version.

Thanks for your help!

Deleting the Facebook application and re-creating it, then configuring my code with the new API key and secret key, resolved this problem.