Confusing error message in 15.6

[Dmitry O Litvintsev]

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}_backup to pg_backup_{start_stop})

Thanks,
Dmitry