I’m developing a Facebook application for fun in my free time and found the solution to a problem that nagged me for a while.
To develop a Facebook application, you use the Facebook Developer application and create a new application. You can even define a localhost application so that you can debug your application locally. Once all the required parameters are well defined for you application and you begin to test the Facebook integration, you may encounter the following error when following you Canvas Page URL:
When this was happening to me, my application wasn’t being called at all by Facebook. If you have a real error in your application, your application should be called and you should be able to debug it or you should be at least getting a 404 error from your server.
With trial and error, incomprehension and a lot of swearing, I finally found out that this is a bug in Facebook and pinpointed exactly how it happens for me.
Ensure that the Canvas Page URL and that your Side Nav URL are both in lowercase! There is something in the Facebook folks code that forbids you to add any uppercase letters to the Canvas Page URL and Side Nav URL. So, to be on the safe side, for every URL defined in your application’s settings in Facebook, use lowercase letters!
By the way, it now looks like this problem is now specific to the Side Nav URL being in lowercase. I am pretty sure that there was a similar Facebook error when the Canvas Page URL was in uppercase.