"Extra keys limit has exceeded"

"Extra keys limit has exceeded"

Hello,

Has anyone encountered the below message before in a workflow log failure: "Update Record Response is {\"code\":52,\"message\":\"Extra keys limit has exceeded\"}"

The script is supposed to generate an invoice for a deposit when a deposit payment is made against a Sales Order. It worked originally, but has now stopped. 

The message is generated by the following section of code (mp is the map adding the item 'Deposit' to the Sales Order.

update = zoho.books.updateRecord("salesorders",org_id,so_id,mp,"this_books3");
info "Update Record Response is " + update;

The workflow log also produces the more familiar error message: Check and update the code in line 132 as there is a Exception : Value is empty and 'get' function cannot be applied. The line this refers to is new_sales_order_lines = update.get("salesorder").get("line_items");

I'm familiar with the 'get' function cannot be applied message, but I am mystified by the meaning of 'Extra keys limit has exceeded'.

Thanks,

Matthew