Eh? Should not be if you are using the P series holder with it.
I think you can fit it into the rearmost slot (nearest the lens), it will be held in by the 2 slightly raised ends on the plastic holder.. dunno if that explains it clearly but that's what I do.
Oh yah.. and I think it can only go in from one end.. one of the raised plastic thingys is higher then the other.
Hope this helps.. I'm no expert either.