Z depth value problem

Hi
I’ve a problem with the z value pass. the image renders with no problems but when i run the z value through the normalize node and to the viewer i just get a black circle in the middle. Does anyone know why?
screens:



NWM I found the solution. i had a box placed right in front of the camera for volume scaterring. i edited it so that the camera is inside the box and it works just fineXD