Credentials Invalid data when preloading for Moneris Checkout

I'm having trouble with getting the preloading to work with MonerisCheckout. In PHP, I send a cURL request (https://gatewayt.moneris.com/chkt/request/request.php) with the following body: 
{"store_id":"xxx","api_token":"xxxx","checkout_id":"xxx","txn_total":"50.00","environment":"qa","action":"preload","language":"fr"}
And I get this response: 
{"response" : {"success":"false","error":{"field":"credentials","message":"invalid data"}}}

I did take the credentials from my developer account and from the test site for the MRC.

 

What does this error mean?