If you are trying to register a callback on getting a URL passed to your program through AppleEvents, don’t use the documented kAEInternetSuite and kAEISGetURL enums. They are defined as ‘gurl’, but the actual value passed to your program is ‘GURL’. From what I can tell, everyone defines their own enums because this is well known.
Apple can’t fix it though, because it would break everyone’s software. So they keep documenting it as ‘gurl’, and we keep redefining it to the actual value.