To clarify this is an error returned as part of the REST request, not really related to the SDK. I am able to get the other subcollections, such as a list of the keys, fine. Only the certs subcollection results in this error.
API guide indicates possible reasons for a 400 error are a malformed request or incorrect name in request. I confirmed the name with the parent collection (crypto), and requesting in the same manner as done for the keys subcollection. Not sure what is going wrong with listing the cert subcollection.