Divide by zero error

Hi

I am trying to process this order to an invoice (screen shot attached). When I try to record the invoice I get an error saying the sytem needs to restart. I've clicked the link to find the error is divide by zero. 

The top item on the order was purchased from a supplier specifically for this job so is the same amount that is in stock.

No items are repeated in the invoice, none of the items have a discount applied and all are to two decimal places. 

I've already spent 1 1/2 hours on the phone yesterday to speak to someone, as I could provide our serial number as our office staff had finished for the day, they wouldn't/couldn't help me. I know have our serial number but don't have another 1 1/2 hours to wait to speak to someone.