Attempting to set a nonexistent usermode doesn't cause an error

[Xe]

fluttershy ~ > nc 127.0.0.1 6667
> :fluttershy-tethys.yolo-swag.com NOTICE * :*** Looking up your hostname
> :fluttershy-tethys.yolo-swag.com NOTICE * :*** Found your hostname: localhost
< NICK TEST
< USER a a a a
> :fluttershy-tethys.yolo-swag.com 001 TEST :Welcome to the ShadowNET Internet Relay Chat Network TEST
> :fluttershy-tethys.yolo-swag.com 002 TEST :Your host is fluttershy-tethys.yolo-swag.com, running version Tethys-0.1-alpha1
> :fluttershy-tethys.yolo-swag.com 003 TEST :This server was born on Sun Jun 01 at 22:11:46 UTC
> :fluttershy-tethys.yolo-swag.com 005 TEST PREFIX=(ov)@+ CHANTYPES=#& CHANMODES=beIq,k,fl,cgimnpstz MODES=4 MAXLIST=50 CASEMAPPING=rfc1459 NICKLEN=20 TOPICLEN=250 CHANNELLEN=50 AWAYLEN=256 MAXTARGETS=1 EXCEPTS INVEX FNC WHOX :are supported by this server
> :fluttershy-tethys.yolo-swag.com 422 TEST :MOTD missing
< MODE TEST +R
< MODE TEST -i
< MODE TEST +i
> :TEST MODE TEST +i
< QUIT
> :TEST!a@localhost QUIT :Client Quit