diff --git a/developer/bin/list_running_app_ids b/developer/bin/list_running_app_ids
index 9ba7337315570..087df387d5140 100755
--- a/developer/bin/list_running_app_ids
+++ b/developer/bin/list_running_app_ids
@@ -14,6 +14,7 @@
 
 require "open3"
 require "io/console"
+require "set"
 
 ###
 ### globals