External sub domain does not resolve using domain mapping


I am trying to map an external sub domain to my main site.

I have followed the instructions to add came records fort with and without www and have successfully managed to map the www sub domain however, without the www and the mapping does not resolve - my browser simply points to my external domains server stating I have reached the page in error.

I have checked and the settings for the came are exactly the same for both with www and without.

Any ideas?