Character Names with the phrase "gui" in them are not allowed?

Hey, I'm trying to get a hold of a username for a character in LaTale, but apparently this username is not allowed, I know it hasn't been taken, or if it has, there are definitely other variations of it that I could spell out, but anytime I try to add "gui" in my username, the game says "This nickname is not allowed. Please choose a different nickname." I've tried, guitar, disguise, guild, guidance, etc. nothing is allowed. Please help. I'd rather not type out the username I want for fear of it being taken (though I guess its obvious if you think about it) but I'd like to know why "gui" is not allowed for a name.

I put this under technical support, because this seems to be a bug of some sort, right? Idk maybe it should be under off-topic? I just want some help, please.