Error with "poker" in the URL

Hi,

Today an user has registered a blog in my site called "poker". Then the address was poker.centraldeblogs.com, but this address don´t charge the css, and gives error at enter the control panel.

I have tried many variations e.g. ("blogdepoker", "miblogdepoker", "pruebaspoker"), but if exists the word poker it crashes.
I tested also with poker word variations e.g.("oker", "roker", "poko"), and this works fine.

It´s a Wordpress bug?