PayPal Sandbox Execute Call returning Error 400 Bad Request

In implementing a server Paypal integration, the POST Create call works correctly to https://api.sandbox.paypal.com/v1/payments/payment with the response object matching that found in the API. The server successfully returns the result to the Client, and res.id for the .then(res) function of paypal.request.post is: PAY-76T970067E989851JLPLCMKA

After the user authorizes payment, the client sends a request to my server’s execute endpoint with a payload of:

{     "paymentID" : "PAY-76T970067E989851JLPLCMKA",     "payerID" : "3LSD3Q8J7T3ZL" } 

The server then remodels the payload for a query to paypal’s POST Execute endpoint as follows:

headers:

Content-Type = "application/json" Authorization = "Bearer " + accessToken // checked not expired 

and a body payload of:

{"payer_id" : "3LSD3Q8J7T3ZL"} 

The post is then made to:

https://api.sandbox.paypal.com/v1/payments/payment/PAY-76T970067E989851JLPLCMKA/execute

at which point, the paypal server responds with: The remote server returned an error: (400) Bad Request.

Form what I can see, this matches the API requirements outlined here: https://developer.paypal.com/docs/api/payments/v1/#payment_execute

As a side note, I setup an endpoint on my own server to check the headers and JSON payload being submitted to the Paypal execute endpoint are correct.

Any suggestions would be greatly appreciated.