Update Lookup on custom module

Hi,

I am attempting to update a custom field (lookup) on a custom module. I am looking to create a workflow rule to call a custom function on create. I want to populate the lookup field with the account id, below is the example of the code I am using;

mySuppID=input.SupplyID.toLong();

relatedSupply = zoho.crm.getRecordById("CustomModule2",mySuppID);

//info relatedSupply;

siteid=relatedSupply.get("Site_ID");

mysiteid=siteid.toLong();

relatedSite = zoho.crm.getRecordById("CustomModule1",mysiteid);

//info relatedSite;

accid=relatedSite.get("Account_ID");

mymap=map();

mymap.put(("Account_ID"),accid);

update = zoho.crm.updateRecord("CustomModule7",accid,mymap);

On the update I receive the below error, is there possibly a restriction on updating this field?

I am pretty confident I am using the correct field name as when I pull out the fields from this module where the field ids populated I can see the name.

Any advise would be greatly appreciated, thanks.