diff --git a/testcases/t/166-assign.t b/testcases/t/166-assign.t
index 43338d55..d79c1000 100644
--- a/testcases/t/166-assign.t
+++ b/testcases/t/166-assign.t
@@ -171,6 +171,7 @@ $workspaces = get_workspace_names;
 ok(!("targetws" ~~ @{$workspaces}), 'targetws does not exist yet');
 
 $window = open_special(wm_class => 'SPEcial');
+wait_for_map $window;
 
 $content = get_ws($tmp);
 ok(@{$content->{nodes}} == 0, 'no tiling cons');