I'm having trouble updating systematically updating parent form links according to new subform information.

I have a parent form, FORM A, which has the main information about an organization, address, phone number, etc. On FORM A, I have a subform, FORM B, which would contain transaction information. Each transaction contains a ton of information so I want to be able to add transaction information individually without having to go through FORM A each time.

Because FORM B is also a form on it's own, I made a dropdown such that if the organization already exists, you can simply pick it from the list. This saves the user from having to edit the main list each time and having to deal with the subforms. I wrote code to generate a system of unique REC_LINK_ID's which I want to use to link each transaction.

My trouble is on updating the links. Here is the code I want to implement on completion of FORM B:

Organization = Organization_List[Organization_Name == input.Organization];

Organization.Subform = REC_LINK_ID;

Here is the error I get when I try to update with the code above:

The field Organization.Subform is of type BIGINTLIST. But you are trying to update it with data of type BIGINT

{where "Organization_List" is my FORM A, and "Subform" is the subform on FORM A, REC_LINK_ID is a unique integer associated with each transaction}

Thanks for the help!

Kyle