Gravityforms error "The field cannot be found"

Gravityforms error "The field cannot be found"

I am using GravityForms with the official Zoho CRM Add-On. I have set up a feed to create a new contact on form submission. However, the contact is not created, and my debug log shows:

ERROR --> GFZohoCRM::create_contact(): The field  cannot be found at Zoho CRM. 

This is unhelpful. The only fields I am trying to map are Name (First), Name (Last) and Email.

Any ideas on where to start debugging?