Facebook invite Canvas page does not load

Question

I've implemented Facebook Invite as follows: - http://www.jobwalk.com.au/invite.aspx

You can login via Facebook and go through the steps (no database update) from: - http://www.jobwalk.com.au

The problem is, when I select someone and press Send Invite and in the final stages, it does not load the Callback or Canvas URL. It just has a Facebook page with a Blank Page in its IFRAME.

Facebook application values

• Site URL: http://www.jobwalk.com.au/
• Canvas URL: http://www.jobwalk.com.au/callback/
• Canvas FBML/iframe: iframe

Does anyone know what went wrong here?

Thank you.