Add up values of one field of two forms base on the value of another field

I have two forms:

Form A: Add_product , contains fields of product name, Product description, EC code and sales

Form B: Sales_report, contains fields of EC_code and sales

I want to send the value of sales from Form B to Form A and keep adding up when there is new record, when EC code of two forms are the same.

And here is the code i wrote:

fet=Add_Product[EC_Code == input.EC_Code];

fet.Sales = fet.Sales + input.Sales;

I try to execute the code when the Sales_report is successfully submitted.

However the code doesn't work and returns the error message when i submit Form B:

Error in executing On Add- On success script.

Unable to update the values to fet.Sales. Line:(2)

Null value occurred while performing addition operation: Line:(2)

Can you help?

thanks a lot!