BUG advisory_lock

DECLARE
varUnlocked boolean;
BEGIN
varUnlocked:= pg_try_advisory_lock( 783264760123456 ); -- Try to lock function
IF not varUnlocked THEN return NULL; END IF; -- exit if locking is unsucessfull

WRONG QUERY HERE WHICH INTERRUPT EXECUTION

varUnlocked:= pg_advisory_unlock( 783264760123456 ); -- unlock function
END

NEXT time when I run this function I always get NULL until server reboot

EXPECTED: Pg must unlock if function execution failed