Confusing error message in 15.6

Hi,

I am observing the following error which confuses me:

#  psql -U postgres template1 -c "checkpoint; select pg_backup_start('${dest}.tar.Z', true)"
CHECKPOINT
 pg_backup_start
-----------------
 17BF7/3009498
(1 row)

#  psql -U postgres template1 -c "select pg_backup_stop(true)"
ERROR:  backup is not in progress
HINT:  Did you call pg_backup_start()?

This is postgresql 15.6 running on RH 7.

This is not just amusing, it is breaking my backup script after update from 11 to 15 (and change from
pg_{start,stop}_backupto pg_backup_{start_stop}) 

Thanks,
Dmitry