Subform Fetch/Lookup

I am having a problem setting up a fetch lookup to a form that uses a subform.

I have attached some screenshots to help me explain.

Here is my main form. The BOM subform section is added as I create the MMR(master mfg record)

Once my MMR is setup I will need to create a BPR(batch production record) for each production run that will include everything from the MMR. Here's what I have so far on the BPR form

I have the lookup MMR set up & it works to bring in the Product_Name

But I get an error when I try to bring in the BOM_Material using the code:

fetmmr = Master_Manufacturing_Record[ID == input.Master_Manufacturing_Record_BPR];

Product_Name_BPR=fetmmr.Product_Name;

BOM_Material_BPR=fetmmr.BOM.Material;

The error is telling me that the var is set for string but trying to update to int.

Variable 'BOM_Material_BPR' is already defined of datatype 'STRING' but trying to update 'BIGINT' datatype

I'm assuming it is because the data its fetcing is created with the add new option.

Anyway around this?