STrange effect with collision

I put a camera in a mesh room created with maplet (b3d exported). I move camera (like a first person game) in the room with arrows and i set collision type between camera-room with:

Collisions type_camera,type_myroom,2,2

and radius of camera:

EntityRadius camera,1,1

But... When camera is near a wall the wall's texture disappear, and i can see trough it. What exactly happen? A solution is to increase the radius of camera, but with this trick i cannot walk in corridors...