diff --git a/testcases/lib/StartXServer.pm b/testcases/lib/StartXServer.pm
index 9d4e40c7..49976394 100644
--- a/testcases/lib/StartXServer.pm
+++ b/testcases/lib/StartXServer.pm
@@ -87,7 +87,7 @@ sub start_xserver {
 
     # First get the last used display number, then increment it by one.
     # Effectively falls back to 1 if no X server is running.
-    my ($displaynum) = map { /(\d+)$/ } reverse sort glob($x_socketpath . '*');
+    my ($displaynum) = reverse sort { $a <=> $b } map{ /(\d+)$/ } glob($x_socketpath . '*');
     $displaynum++;
 
     say "Starting $parallel Xephyr instances, starting at :$displaynum...";