Shredder not shredding attachments?

So I’m running validator and getting the following error for a few tickets :

example :

Record #600816 in Attachments references not existent record in Attachments
Parent => ‘600813’ => id
Attachment #600816 -> Txn #1567423, Transaction #1567423 -> object
RT::Ticket #187697

I attempted to fix this using shredder for ticket ID 187697
(succesfully), but am still getting the error. How do I shred the
attachment itself? I’m afraid I’m not sure what the query would be.

Many thanks for any assistance