Unknown OID warnings in Postgresql::ReplicationSlot.lag_too_great?

This appears to happen only on the initial call:

[ gstg ] production> Postgresql::ReplicationSlot.lag_too_great?
unknown OID 28: failed to recognize type of 'xmin'. It will be treated as String.
unknown OID 3220: failed to recognize type of 'restart_lsn'. It will be treated as String.
=> false
[ gstg ] production> Postgresql::ReplicationSlot.lag_too_great?
=> false

/cc: @yorickpeterse