I need some help with gtkradiant.

Hello,
I made a map in gtkradiant and i build it ingame works great but 1 problem there are some places where i dit not create a object and i can stil stand on it. Its is invisible its like a bug how to fix it?

EDIT :

fixed the problem whas easy i copyed somting and turnd it not good enof so it whas not right that makes the bug i made it right bugs gone and thansk for the info hellsniper