diff --git a/lib/models/room-state.js b/lib/models/room-state.js
index 0044ef65a..765c96c86 100644
--- a/lib/models/room-state.js
+++ b/lib/models/room-state.js
@@ -206,15 +206,19 @@ function _updateDisplayNameCache(roomState, userId, displayName) {
     var oldName = roomState._userIdsToDisplayNames[userId];
     delete roomState._userIdsToDisplayNames[userId];
     if (oldName) {
-        var existing = roomState._displayNameToUserIds[oldName] || [];
-        for (var i = 0; i < existing.length; i++) {
-            if (existing[i] === userId) {
+        // Remove the old name from the cache.
+        // We clobber the user_id > name lookup but the name -> [user_id] lookup
+        // means we need to remove that user ID from that array rather than nuking
+        // the lot.
+        var existingUserIds = roomState._displayNameToUserIds[oldName] || [];
+        for (var i = 0; i < existingUserIds.length; i++) {
+            if (existingUserIds[i] === userId) {
                 // remove this user ID from this array
-                existing.splice(i, 1);
+                existingUserIds.splice(i, 1);
                 i--;
             }
         }
-        roomState._displayNameToUserIds[oldName] = existing;
+        roomState._displayNameToUserIds[oldName] = existingUserIds;
     }
 
     roomState._userIdsToDisplayNames[userId] = displayName;