diff --git a/lib/src/ua.dart b/lib/src/ua.dart
index 8d3626da..25ff532e 100644
--- a/lib/src/ua.dart
+++ b/lib/src/ua.dart
@@ -317,7 +317,7 @@ class UA extends EventManager {
     int num_sessions = _sessions.length;
 
     // Run  _terminate_ on every Session.
-    _sessions.forEach((String? key, _) {
+    _sessions.keys.toList().forEach((String? key, _) {
       if (_sessions.containsKey(key)) {
         logger.d('closing session $key');
         try {