Invalid Recipient Auth Method Error When Sending Agreements via Apex

I'm using Apex to automate sending Adobe Sign agreements. My approach:

1. Call AgreementTemplateService.load(templateId, contactId) to generate agreement IDs for a list of contacts.
2. Pass these IDs to EchoSignApiService.sendDocument(agreementId) to send the PDFs.

This runs in a Queueable job triggered by a button click in Salesforce. I successfully get agreement IDs, but when sending, I receive:
null :400 {code:INVALID_RECIPIENT_AUTH_METHOD, message:Invalid authentication method for recipient}