Shimano 600 pedals and toe clip question.

I must be missing something. When I put my foot into the right pedal strap a lot of times the stap slips out of the slot that it goes through. My Shimano 105 pedals and toe clips have an unopen slots that the strap goes though so there is no way it can come out. Am I missing something? Here a photo of the pedal and the slot that seems to always come undone.