Writer Merge and Sign OAUTH API Call error.

Writer Merge and Sign OAUTH API Call error.

Trying to send a request to the Merge and Sign API. I keep getting "Merge data not found. Please try again with proper merge data." Request is sent via JS axios call. We use the same exact data inside a CRM function and everything works. 
Followed the documentation here: https://www.zoho.com/writer/help/api/v1/merge-and-sign.html

Below is the request that's sent. 
  2. {
  3.   method: 'POST',
  4.   url: 'https://www.zohoapis.com/writer/api/v1/documents/[DOCUMENT ID]/merge/sign',
  5.   body: {
  6.     service_name: 'zohosign',
  7.     filename: '[FILE NAME],
  8.     merge_data: { data: [Array] }
  9.   },
  10.   headers: {
  11.     Authorization: 'Zoho-oauthtoken [ACCESS TOKEN]'
  12.   }
  13. }