Single Kelight Frame Black Error

Hi everyone I think I am having a bug which I had fixed in the past in another comp but can't seem to do it here and not sure if I remeber correctly how I fixed it last time. 

The problem is after keylight 1.2 applied and tuned there is a random black frame applied to the keylight clip. I thought it might be the black and white matte settings but altering those doesn't get rid of it. It's only one frame out of hundreds. Any clues appreciated that's so much.