left expression is of type BIGINT and right expression is of type STRING and the operator == is not valid (Deluge error)

left expression is of type BIGINT and right expression is of type STRING and the operator == is not valid (Deluge error)

Hi,
I want to fetch records from my Dispatch and update Stock but seems to have problem in the field type and the field type is supposed to be String 

This is my code:

for each Stock_Record_Obj in input.Fabric_Details
{
product_Obj = Fabric_Stock[ID == Stock_Record_Obj.Design_No];
product_Obj.Meter = product_Obj.Meter - Fabric_Details.Meter;
}