Can't register a name

I use the user name "kitsunisan" on all forums I go to, but when I tried here, it tells me the user name is either in use or is not acceptable. So, figuring someone beat me to it, I use the name I currently have. When I get in, just out of curiosity, I search for the name kitsunisan, and I get the message "Invalid User specified". If there is no kitsunisan on the forums, why was I not able to use the name? Not really a big deal, I just know its going to take me a while to remember the name I have on the board when I log in, and if its not being used, I'd like to have it.