"document.thisform.num1.checked" is null or not an object

I have a pretty basic form on my site and I'm trying to calculate the total cost based on the number of tickets needed in several categories. I keep getting this error...

"document.thisform.num1.checked" is null or not an object

I know its a simple error but I can't seem to isolate the problem. Any help would be greatly appreciated.

To view it online its at www.webmarkart.com/sccc_theatre/default.asp?action=bo

or download the code here...

I don't know if this is the solution to the problem, or if I'm just bringing attention to another one -- but JavaScript is case-sensitive.

You're calculating
document.thisform.num1.value

but your input box is named
<INPUT TYPE="CHECKBOX" NAME="NUM1" VALUE="0.00">

Thank you! that was it!