?_= in URL...?

[eluser]Unknown[/eluser]
Hi

I'm using a tool-tip library. (qTip I believe) I would like to use the non-caching version which simply add ?_=1298471294712 to the end of the URL. Why does that not work through codeigniter?

I have made sure all the characters are allowed and

http://blabla.com/_=1298471294712
http://blabla.com/?=1298471294712
http://blabla.com/?_1298471294712

works fine but not:
http://blabla.com/?_=1298471294712

What am I not seeing...?

Thanks in advance - and have a nice day