When trying to progress to Step 2 of my store checkout, I'm getting the following message:

Fatal error: Cannot instantiate non-existent class: ot_insurance in /home3/mindy/kings-kreations-www/zen/includes/classes/order_total.php on line 36

Here is line 36: $GLOBALS[$class] = new $class;

I had been working on making some changes to my shipping insurance module tonight. I wasn't even working in the area that is giving me the error message. I re-uploaded the order_total.php file from my backup disc made about a week ago. No change to the error. I have not made any other changes or upgrades. I am using version 1.3.7. My site is www.kings-kreations.com/zen.

Can someone PLEASE help?!?!