Skateboarding ollie help!?

Ok so I can nearlly do an olly its just that i have one problem the board is turning 90 degrees in the air while im doing it. I took a video and watched it back in slow motion and i think i know what the problem is i just cant fix it. The problem is as im sliding my front foot up the board my knee is twisting which is twisting the foot which is twisting the board i cant seem to stop my knee from twisting so its facing the nose of the board rather than the toe side of the board. Any help?