Well, you have this nice bit of code/game for which one part doesn't work. What I'm suggesting is that you reduce it to it's bare components and test the componenet that is failing apart from the other componenets.

In other words, from scratch, program the collision of a physics entity with a non-physics entity and nothing else. See if this simple test reproduces the errors that you are experiencing or not. This will let you know if the problem is truly within the physics engine or if it's with your script...and since no one can reproduce this error, my guess is that it's with your script.