Problem with registration hook

[MarkPW]

I'm attempting to add a small plug-in to the register_addmember_process hook so that I can check for invalid characters in usernames.

I've put together the following code, however when testing on the registration page the phrase (which I have added as a custom phrase) is not retrieved and is returned as... "Could not find phrase 'm24_invalidusername'". I've also tested using default phrases so I know there isn't a problem with my custom phrase, and I receive the same error.

PHP Code:

if(!ereg("^[a-zA-Z0-9_-]+$",$vbulletin->GPC['username']))
{
$userdata->error('m24_invalidusername');
} 


Would anyone know why this isn't working?