xbindkeys doesn't react on Ctrl+Alt+KP_Add
 

I used xbindkeys_config to configure xbindkeys to start a command on Ctrl+Alt+KP_Add press. It doesn't react. I start it as "xbindkeys -n -v", and it says something when i press other key combos, but doesn't say anything on Ctrl+Alt+KP_Add press. I guess it has something to do with the legacy way of changing screen resolutions in Xorg. But nowadays this key combo isn't used anymore for this, how can i set up it to my needs?