diff --git a/lib/SyTest/Homeserver/Synapse.pm b/lib/SyTest/Homeserver/Synapse.pm
index 264be18c2..23b60346a 100644
--- a/lib/SyTest/Homeserver/Synapse.pm
+++ b/lib/SyTest/Homeserver/Synapse.pm
@@ -265,7 +265,9 @@ sub start
         send_federation       => ( not $self->{workers} ),
         enable_media_repo     => ( not $self->{workers} ),
         run_background_tasks_on  => ( $self->{workers} ? "background_worker1" : "master" ),
-        worker_to_update_user_directory  => ( $self->{workers} ? "user_dir" : "null" ),
+        $self->{workers} ? (
+            update_user_directory_on  => "user_dir",
+        ) : (),
         # update_user_directory is kept for backwards compatibility,
         # worker_to_update_user_directory is prioritized before this option.
         update_user_directory => ( not $self->{workers} ),