Getting missing-change-type Error during Delta Import on Call-based ECMA2

I am developing a call-based ECMA2.  The connected system supports delta import, and has a change type column called "change_type".  It is unclear to me how to properly tell the MA what the change-type column is called.  I had thought all I would need to do is something along these lines:

//Determine the ChangeType.
switch (row["change_type"].ToString())
{
    case "A":
        csentry.ObjectModificationType = ObjectModificationType.Add;
        break;
    case "C":
        csentry.ObjectModificationType = ObjectModificationType.Update;
        break;
    case "D":
        csentry.ObjectModificationType = ObjectModificationType.Delete;
        break;
}

...however after I perform a Delta Import, the MA is processing two "update" records (as noted correctly under Staging statistics), but I get discovery errors for missing-change-type for both.

Can someone please advise on how to properly reflect the change_type within a call-based ECMA2?

Thanks!

Jon