Thin black lines on top and bottom of frame

I ran a test roll (Ilford 50ASA) through my K88 last week. There are very
thin (about 1mm) black lines at the top and bottom of every frame. The
ends of the lines are beveled inwards, towards the center of the frame. I
assume this is a light leak problem of some sort. The shutter seems to
work as designed, so I suspect a film back leak. Is this a logical
conclusion? I don't see any obvious problems with the back... maybe it is
time for a CLA.