Problem with Lookup ID, BIGINT and STRING == not valid

Problem with Lookup ID, BIGINT and STRING == not valid

Dear Sir,

I am not a programmer and have been trying hard to work with Zoho creator. I am working on an app to calculate a total process cost for a work.

Form 1 : Work_Order

Work Order # (single line)
Work Completed? (decision) - if this is "true" then run script below:
Total Process Cost (number) - wish to calculate and return a total based on related values in Production Form

if input.Completed = true
//
{
Total_Process_Cost = 0;
for each r in Production [ID == input.Work_Order]
{
Total_Process_Cost  =  Total_Process_Cost  + r.Process_Cost;
}
}
else
{
Total_Process_Cost  = 0;
}

Form 2 : Production

Work Order # (lookup from Work_Order Form)
Process Cost A (number)
Process Cost B (number)
Process Cost C (number)
Process Cost D (number)

* Different type of work order has different type of processes, for example work order 1 (with process A, C, D) and work order 2 (with process B, D), etc.

I wish to create a field to return a total of process cost according to completed work order. however, when I used the script above, it says:

In Criteria left expression is of type BIGINT and right expression is of type STRING and the operator == is not valid

Could anyone help on this?