diff --git a/lib/src/client/impl/channel_impl.dart b/lib/src/client/impl/channel_impl.dart
index bdfe944..6532467 100644
--- a/lib/src/client/impl/channel_impl.dart
+++ b/lib/src/client/impl/channel_impl.dart
@@ -701,7 +701,8 @@ class _ChannelImpl implements Channel {
     }
 
     // Multi Ack/Nack; messages up to seqNo
-    for (var pendingSeqNo in _pendingDeliveries.keys) {
+    var pdkeys = _pendingDeliveries.keys.toList();
+    for (var pendingSeqNo in pdkeys) {
       if (pendingSeqNo > seqNo) {
         // only interested in keys up to pendingSeqNo
         break;