If you added it later, that probably is the problem. If you picked up stitches rather than using the 'live' live stitches, that would make that happen. if that is the case you can fix by unraveling the ribbing, then pick out the bottom to get at the live stitches and rework the ribbing. It is hard to say the exact problem without actually seeing it.
I wAS GOING TO SAY THE SAME THING.
THE ONLY THING IS UNDOING A CaST On IS WELL NIGH IF NOT IMPOSSIBLE (sorry just noticed pushed caps lock!). So you need to undo above the cast on and pick up the live stitches (assuming you worked it bottom up. Now if you did it top down undoing the cast off is not an issue).
And welcome to KP.