Drawing disappears when I put it in a deformer... HELP!

Im doing a character rig, but when i put a deformer in the left arm and try to move it, BAM! the drawing disappears. The thing is that the arm seems to be there, because when I change the camera to render mode (or when I disable the Enable Deformation button…) I can see the arm… Weird…
I hope you can help me with this issue.
Thanks.
Paul.