diff --git a/bin/utils.js b/bin/utils.js
index 89d226f..3f8e9cc 100644
--- a/bin/utils.js
+++ b/bin/utils.js
@@ -72,8 +72,10 @@ class WSSharedDoc extends Y.Doc {
       const changedClients = added.concat(updated, removed)
       if (conn !== null) {
         const connControlledIDs = /** @type {Set<number>} */ (this.conns.get(conn))
-        added.forEach(clientID => { connControlledIDs.add(clientID) })
-        removed.forEach(clientID => { connControlledIDs.delete(clientID) })
+        if (connControlledIDs !== undefined) {
+          added.forEach(clientID => { connControlledIDs.add(clientID) })
+          removed.forEach(clientID => { connControlledIDs.delete(clientID) })
+        }
       }
       // broadcast awareness update
       const encoder = encoding.createEncoder()