postgres 8.4.20

With these settings, the daemon starts :

> postgresql.conf

> shared_preload_libraries = 'auto_explain'


With these settings, the daemon doesn't start :

> postgresql.conf

> shared_preload_libraries = 'auto_explain'

> auto_explain.log_min_duration = 1000


It is a server with a legacy application, so we cannot upgrade PG.
I cannot find documentation of this behaviour / bug...