The code in pg_stat_get_subscription() appears to believe that it can return a set of rows, but its pg_proc entry does not have proretset set. It may be that this somehow accidentally fails to malfunction when the function is used via the system views, but if you try to call it directly it falls over:
regression=# select pg_stat_get_subscription(0);
ERROR: set-valued function called in context that cannot accept a set
regards, tom lane
