diff --git a/Tapeti/Connection/TapetiWorker.cs b/Tapeti/Connection/TapetiWorker.cs
index f9d577a..df0f3d7 100644
--- a/Tapeti/Connection/TapetiWorker.cs
+++ b/Tapeti/Connection/TapetiWorker.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
@@ -37,6 +38,9 @@ namespace Tapeti.Connection
         private IModel channelInstance;
         private ulong lastDeliveryTag;
         private DateTime connectedDateTime;
+
+        // These fields must be locked, since the callbacks for BasicAck/BasicReturn can run in a different thread
+        private readonly object confirmLock = new Object();
         private readonly Dictionary<ulong, ConfirmMessageInfo> confirmMessages = new Dictionary<ulong, ConfirmMessageInfo>();
         private readonly Dictionary<string, ReturnInfo> returnRoutingKeys = new Dictionary<string, ReturnInfo>();
 
@@ -208,6 +212,9 @@ namespace Tapeti.Connection
                 async (handler, next) => await handler.Handle(context, next),
                 () => taskQueue.Value.Add(async () =>
                 {
+                    if (Thread.CurrentThread.ManagedThreadId != 3)
+                    Debug.WriteLine(Thread.CurrentThread.ManagedThreadId);
+
                     var body = messageSerializer.Serialize(context.Message, context.Properties);
 
                     Task<int> publishResultTask = null;
@@ -225,7 +232,16 @@ namespace Tapeti.Connection
                         {
                             lastDeliveryTag++;
 
-                            confirmMessages.Add(lastDeliveryTag, messageInfo);
+                            Monitor.Enter(confirmLock);
+                            try
+                            {
+                                confirmMessages.Add(lastDeliveryTag, messageInfo);
+                            }
+                            finally
+                            {
+                                Monitor.Exit(confirmLock);
+                            }
+
                             publishResultTask = messageInfo.CompletionSource.Task;
                         }
                         else
@@ -326,7 +342,17 @@ namespace Tapeti.Connection
                     if (config.UsePublisherConfirms)
                     {
                         lastDeliveryTag = 0;
-                        confirmMessages.Clear();
+
+                        Monitor.Enter(confirmLock);
+                        try
+                        {
+                            confirmMessages.Clear();
+                        }
+                        finally
+                        {
+                            Monitor.Exit(confirmLock);
+                        }
+
                         channelInstance.ConfirmSelect();
                     }
 
@@ -403,35 +429,51 @@ namespace Tapeti.Connection
 
         private void HandleBasicAck(object sender, BasicAckEventArgs e)
         {
-            foreach (var deliveryTag in GetDeliveryTags(e))
+            Monitor.Enter(confirmLock);
+            try
             {
-                if (!confirmMessages.TryGetValue(deliveryTag, out var messageInfo)) 
-                    continue;
-
-                if (returnRoutingKeys.TryGetValue(messageInfo.ReturnKey, out var returnInfo))
+                foreach (var deliveryTag in GetDeliveryTags(e))
                 {
-                    messageInfo.CompletionSource.SetResult(returnInfo.FirstReplyCode);
+                    if (!confirmMessages.TryGetValue(deliveryTag, out var messageInfo))
+                        continue;
 
-                    returnInfo.RefCount--;
-                    if (returnInfo.RefCount == 0)
-                        returnRoutingKeys.Remove(messageInfo.ReturnKey);
+                    if (returnRoutingKeys.TryGetValue(messageInfo.ReturnKey, out var returnInfo))
+                    {
+                        messageInfo.CompletionSource.SetResult(returnInfo.FirstReplyCode);
+
+                        returnInfo.RefCount--;
+                        if (returnInfo.RefCount == 0)
+                            returnRoutingKeys.Remove(messageInfo.ReturnKey);
+                    }
+
+                    messageInfo.CompletionSource.SetResult(0);
+                    confirmMessages.Remove(deliveryTag);
                 }
-
-                messageInfo.CompletionSource.SetResult(0);
-                confirmMessages.Remove(deliveryTag);
+            }
+            finally
+            {
+                Monitor.Exit(confirmLock);
             }
         }
 
 
         private void HandleBasicNack(object sender, BasicNackEventArgs e)
         {
-            foreach (var deliveryTag in GetDeliveryTags(e))
+            Monitor.Enter(confirmLock);
+            try
             {
-                if (!confirmMessages.TryGetValue(deliveryTag, out var messageInfo)) 
-                    continue;
+                foreach (var deliveryTag in GetDeliveryTags(e))
+                {
+                    if (!confirmMessages.TryGetValue(deliveryTag, out var messageInfo)) 
+                        continue;
 
-                messageInfo.CompletionSource.SetCanceled();
-                confirmMessages.Remove(e.DeliveryTag);
+                    messageInfo.CompletionSource.SetCanceled();
+                    confirmMessages.Remove(e.DeliveryTag);
+                }
+            }
+            finally
+            {
+                Monitor.Exit(confirmLock);
             }
         }