X-Frame-Options: Post on Twitter from Facebook application

Question

I want to be able to post on users Twitter feed from a Facebook app i'm developing at the moment but seem to be having trouble. When I call my the tmhOAuth PHP script to authenticate ask for permissions on Twitter I get the following error:

Refused to display document because display forbidden by X-Frame-Options

TmhOAuth works perfectly when its called directly from the browser. Its the header function that fails inside the Facebook iframe.

Any idea whats going on?