Only thing I can think that could cause that error is if the bot can't contact the website somehow, so it's leaves the variables undefined rather than empty. Probably a firewall getting in the way at one of the ends.
I dont know if anyone is even using this script or updating it anymore but I tried both urban.tcl and urban2.tcl from Trixar_za and all i get is "Nothing found for "
I get no errors anywhere it just returns no results from urban dictionary.