convertToNodeSpaceAR problem

I had a game that was mostly working, then I decided to clean up my coordinate space related code, now that I understand Cocos2D better. However, the sprites now all flicker, even when nothing is moving. The cause is that on alternate world updates, convertToNodeSpaceAR is returning (0,0) instead of the correct coordinate. I’ve also tried convertToNodeSpace, which does the same thing.

What I am doing is syncronizing my sprites to the box2d bodies, during the call to update() of one of my CCLayers.