conversion to double impossible

1
hi,

I'm trying to do a priming experiment, it works well, but if subjects
press two keys to respond, it crashs.
I would like to know if someone is able to understand what happens,
I can't solve this problem because I can't even know what means the
error message:

??? The following error occurred converting from cell to double:
Error using ==> double
Conversion to double from cell is not possible.

Thanks.

PIERRE.