I have been playing with innerHTML recently and I know I have seen and conquered this error on several occassions but this time I just cannot find it. Anytime you click on a square it gives the error and then the location of the <td> tag you clicked on. WHY!!

I spelled the function correctly, and I placed the value in correctly, so what is going on?

http://cpradio.net/tic-tac-toe.html