diff --git a/src/main/csharp/MessageConsumer.cs b/src/main/csharp/MessageConsumer.cs
index 3f9fd4d..ca44893 100755
--- a/src/main/csharp/MessageConsumer.cs
+++ b/src/main/csharp/MessageConsumer.cs
@@ -751,7 +751,7 @@
 	                            }
 	                            foreach (MessageDispatch delivered in dispatchedMessages) 
 								{
-	                                this.previouslyDeliveredMessages.Add(delivered.Message.MessageId, false);
+	                                this.previouslyDeliveredMessages[delivered.Message.MessageId] = false;
 	                            }
 	                        } 
 							else 
@@ -822,7 +822,7 @@
 								{
 	                                if (previouslyDeliveredMessages != null) 
 									{
-	                                    previouslyDeliveredMessages.Add(dispatch.Message.MessageId, true);
+	                                    previouslyDeliveredMessages[dispatch.Message.MessageId] = true;
 	                                } 
 									else 
 									{