Error on negative % gross profit on standard netsuite script

Have incorporated the following script to figure GP on line items within SO transactions, I am getting error messages when Margins go negative.  Not knowing much about coding I thought someone here might be willing to give me some guidance....

Thanks, appreciate any help.

/* -------------------------------------------------------------------------------------------------------
*
Function: grossProfitPercent

Functionality:
Calculates gross profit percent at line item level.

Deployment:
The script can be deployed on Sales Order.

Event triggering the script:
1) validate Line.

Custom Fields:
+------------------------------------------------------------------------------------------------------+
| S. No.| Id     | Type   | Level |  Description      
+------------------------------------------------------------------------------------------------------+
| (1) | _purchaseprice  | Currency  | Column | It holds purchase price sourced from
                 item master        |
| (2) | _grossprofitpercent | Percent   | Column | Holds gross profit percent calculated |