diff --git a/OpenRA.Game/Map/MapCache.cs b/OpenRA.Game/Map/MapCache.cs
index c75eaa8eee..c7beec372d 100644
--- a/OpenRA.Game/Map/MapCache.cs
+++ b/OpenRA.Game/Map/MapCache.cs
@@ -149,7 +149,7 @@ namespace OpenRA
 						{
 							LastModifiedMap = uid;
 							if (oldMap != null)
-								mapUpdates.Add(oldMap, uid);
+								mapUpdates[oldMap] = uid;
 						}
 					}
 				}