diff --git a/OpenSim/Framework/ClientManager.cs b/OpenSim/Framework/ClientManager.cs
index 51ebc9b324..3df883d44b 100644
--- a/OpenSim/Framework/ClientManager.cs
+++ b/OpenSim/Framework/ClientManager.cs
@@ -72,20 +72,28 @@ namespace OpenSim.Framework
         public void Remove(uint id)
         {
             //m_log.InfoFormat("[CLIENT]: Removing client with code {0}, current count {1}", id, m_clients.Count);            
-            m_clients.Remove(id);
+            lock (m_clients)
+            {
+                m_clients.Remove(id);
+            }
             m_log.InfoFormat("[CLIENT]: Removed client with code {0}, new client count {1}", id, m_clients.Count);
         }
 
         public void Add(uint id, IClientAPI client)
         {
-            m_clients.Add(id, client);
+            lock (m_clients)
+            {
+                m_clients.Add(id, client);
+            }
         }
 
         public void InPacket(uint circuitCode, Packet packet)
         {
             IClientAPI client;
-
-            if (m_clients.TryGetValue(circuitCode, out client))
+            bool tryGetRet = false;
+            lock (m_clients)
+                tryGetRet = m_clients.TryGetValue(circuitCode, out client);
+            if(tryGetRet)
             {
                 client.InPacket(packet);
             }
@@ -94,8 +102,10 @@ namespace OpenSim.Framework
         public void CloseAllAgents(uint circuitCode)
         {
             IClientAPI client;
-
-            if (m_clients.TryGetValue(circuitCode, out client))
+            bool tryGetRet = false;
+            lock (m_clients)
+                tryGetRet = m_clients.TryGetValue(circuitCode, out client);
+            if (tryGetRet) 
             {
                 CloseAllCircuits(client.AgentId);
             }
@@ -111,8 +121,10 @@ namespace OpenSim.Framework
                 IClientAPI client;
                 try
                 {
-
-                    if (m_clients.TryGetValue(circuits[i], out client))
+                    bool tryGetRet = false;
+                    lock (m_clients)
+                        tryGetRet = m_clients.TryGetValue(circuits[i], out client);
+                    if(tryGetRet)
                     {
                         Remove(client.CircuitCode);
                         client.Close(false);
@@ -176,7 +188,10 @@ namespace OpenSim.Framework
 
         public bool TryGetClient(uint circuitId, out IClientAPI user)
         {
-            return m_clients.TryGetValue(circuitId, out user);
+            lock (m_clients)
+            {
+                return m_clients.TryGetValue(circuitId, out user);
+            }
         }
     }
 }
diff --git a/OpenSim/Region/ClientStack/UDPServer.cs b/OpenSim/Region/ClientStack/UDPServer.cs
index 7b32490803..19974531b1 100644
--- a/OpenSim/Region/ClientStack/UDPServer.cs
+++ b/OpenSim/Region/ClientStack/UDPServer.cs
@@ -311,9 +311,12 @@ namespace OpenSim.Region.ClientStack
         private void CloseEndPoint(EndPoint sender)
         {
             uint circuit;
-            if (clientCircuits.TryGetValue(sender, out circuit))
+            lock (clientCircuits)
             {
-                m_packetServer.CloseCircuit(circuit);
+                if (clientCircuits.TryGetValue(sender, out circuit))
+                {
+                    m_packetServer.CloseCircuit(circuit);
+                }
             }
         }
 
@@ -322,12 +325,17 @@ namespace OpenSim.Region.ClientStack
             UseCircuitCodePacket useCircuit = (UseCircuitCodePacket) packet;
             lock (clientCircuits)
             {
-                clientCircuits.Add(epSender, useCircuit.CircuitCode.Code);
+                if (!clientCircuits.ContainsKey(epSender))
+                    clientCircuits.Add(epSender, useCircuit.CircuitCode.Code);
+                else
+                    m_log.Error("[UDPSERVER]: clientCircuits already contans entry for user " + useCircuit.CircuitCode.Code.ToString() + ". NOT adding.");
             }
             lock (clientCircuits_reverse)
             {
                 if (!clientCircuits_reverse.ContainsKey(useCircuit.CircuitCode.Code))
-                clientCircuits_reverse.Add(useCircuit.CircuitCode.Code, epSender);
+                    clientCircuits_reverse.Add(useCircuit.CircuitCode.Code, epSender);
+                else
+                    m_log.Error("[UDPSERVER]: clientCurcuits_reverse already contains entry for user " + useCircuit.CircuitCode.Code.ToString() + ". NOT adding.");
             }
 
             PacketServer.AddNewClient(epSender, useCircuit, m_assetCache, m_authenticateSessionsClass);
@@ -380,22 +388,28 @@ namespace OpenSim.Region.ClientStack
         {
             // find the endpoint for this circuit
             EndPoint sendto = null;
-            if (clientCircuits_reverse.TryGetValue(circuitcode, out sendto))
+            lock (clientCircuits_reverse)
             {
-                //we found the endpoint so send the packet to it
-                Server.SendTo(buffer, size, flags, sendto);
+                if (clientCircuits_reverse.TryGetValue(circuitcode, out sendto))
+                {
+                    //we found the endpoint so send the packet to it
+                    Server.SendTo(buffer, size, flags, sendto);
+                }
             }
         }
 
         public virtual void RemoveClientCircuit(uint circuitcode)
         {
             EndPoint sendto = null;
-            if (clientCircuits_reverse.TryGetValue(circuitcode, out sendto))
+            lock (clientCircuits_reverse)
             {
-                clientCircuits.Remove(sendto);
+                if (clientCircuits_reverse.TryGetValue(circuitcode, out sendto))
+                {
+                    clientCircuits.Remove(sendto);
 
 
-                clientCircuits_reverse.Remove(circuitcode);
+                    clientCircuits_reverse.Remove(circuitcode);
+                }
             }
         }
     }