Console App - Deleting records error

I have a console app for cleaning up attachments in our CRM that for some reason is throwing an error today.

My relevant code is (I pass an Entity Collection of ActivityMimeAttachment records):

foreach (Entity r in records.Entities)
    service.Delete(r.LogicalName, r.Id);

On the delete line I get the error "Guid should contain 32 digits with 4 dashes (xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx)." but the guid is 10f316ee-7a60-e511-8117-c4346bac3e84:

How do I resolve this?

This is for CRM Online version 8.2 - could that have something to do with it?

Thanks

EDIT:

Further testing indicates this is specific to deleting attachments. I can delete other types of records by passing a different entity collection to the foreach loop