Get Record by ID: Variable 'Product_ID' is not defined

Get Record by ID: Variable 'Product_ID' is not defined

I am making an app that uses the following code to record products from the Products module in CRM

But I get this error: Error at line number : 1 
                               Variable 'Product_ID' is not defined
  1. crmResp = zoho.crm.getRecordById("Products", input.Product_ID);
  2. input.Unit_Price = (crmResp.get("Unit Price")).toDecimal();
  3. input.Quantity_Available = (crmResp.get("Qty in Stock")).toLong();