diff --git a/world/zonelist.h b/world/zonelist.h
index 085ba149a..e9f643ea6 100644
--- a/world/zonelist.h
+++ b/world/zonelist.h
@@ -9,6 +9,7 @@
 #include <vector>
 #include <memory>
 #include <deque>
+#include <mutex>
 
 class WorldTCPConnection;
 class ServerPacket;