I've looked this error up, and it seems to be related to inserting updating records. I'm not trying to do anything like that. I'm simply using
Messaging.sendEmail(). I'm passing it a list of
Messaging.SingleEmailMessage. When debugging it appears that this list is properly populated. What could be causing this error when trying to send messages?
If it helps at all I'm calling this through a Rest API endpoint I've written in a without sharing class.