Of course it is about the script. Why don't you take a look at the debug output. And there's no nick registration on Undernet.
Here are the step-by-step instructions to reproduce that error message:
1. Try to connect to any Undernet server. (I didn't test it on other networks)
2. Send any command to the server while you are loggin on to the server. I mean while you are still receiving the notices like "*** Looking up your hostname" "*** Checking Ident" etc. For example type /whois nick . The server sends a numeric 451 * :Register first.
The word "Register" doesn't mean to register a nickname or username.
