Bigin to Mailchimp using Zoho Flow

I am trying to make a basic connection to Mailchimp from Bigin by Zoho. The connection is "Contact Created" > "Add or update subscriber". All connections are working fine between Bigin > Zoho Flow and Zoho Flow > Mailchimp.

I thought it was because some fields and null/empty, however these are not "required" fields in Mailchimp, so should not matter that they are empty/null.

The error is: Mailchimp says "The resource submitted could not be validated. For field-specific details, see the 'errors' array."

However no "'errors' array" is provided?

Here is an example of the code submitted to Mailchimp from Bigin (with personal details removed):

{

"Address address line 1": "",

"Company": {

"name": "Company Name Ltd",

"id": 319998000000279668

"Address address line 2": "",

"Subscriber Status": "subscribed",

"First Name": "Firstname",

"Subscriber email": "name@company-name.co.uk",

"Address state": "",