diff --git a/OpenSim/Region/CoreModules/Framework/UserManagement/UserManagementModule.cs b/OpenSim/Region/CoreModules/Framework/UserManagement/UserManagementModule.cs
index 1e70b849c2..ac21b53bd6 100644
--- a/OpenSim/Region/CoreModules/Framework/UserManagement/UserManagementModule.cs
+++ b/OpenSim/Region/CoreModules/Framework/UserManagement/UserManagementModule.cs
@@ -56,6 +56,7 @@ namespace OpenSim.Region.CoreModules.Framework.UserManagement
 
         // The cache
         protected Dictionary<UUID, UserData> m_UserCache = new Dictionary<UUID, UserData>();
+        private object m_AddUserLock = new object();
 
         #region ISharedRegionModule
 
@@ -475,9 +476,11 @@ namespace OpenSim.Region.CoreModules.Framework.UserManagement
 
             UserData oldUser;
             //lock the whole block - prevent concurrent update
-            lock (m_UserCache)
+            lock (m_AddUserLock)
             {
-                m_UserCache.TryGetValue (id, out oldUser);
+                lock (m_UserCache)
+                    m_UserCache.TryGetValue(id, out oldUser);
+
                 if (oldUser != null)
                 {
                     if (creatorData == null || creatorData == String.Empty)
@@ -488,9 +491,10 @@ namespace OpenSim.Region.CoreModules.Framework.UserManagement
 
                     //try update unknown users
                     //and creator's home URL's
-                    if ((oldUser.FirstName == "Unknown" && !creatorData.Contains ("Unknown")) || (oldUser.HomeURL != null && !creatorData.StartsWith (oldUser.HomeURL)))
+                    if ((oldUser.FirstName == "Unknown" && !creatorData.Contains("Unknown")) || (oldUser.HomeURL != null && !creatorData.StartsWith(oldUser.HomeURL)))
                     {
-                        m_UserCache.Remove (id);
+                        lock (m_UserCache)
+                            m_UserCache.Remove(id);
                         m_log.DebugFormat("[USER MANAGEMENT MODULE]: Re-adding user with id {0}, creatorData [{1}] and old HomeURL {2}", id, creatorData, oldUser.HomeURL);
                     }
                     else
@@ -500,38 +504,38 @@ namespace OpenSim.Region.CoreModules.Framework.UserManagement
                     }
                 }
 
-                UserAccount account = m_Scenes[0].UserAccountService.GetUserAccount (m_Scenes [0].RegionInfo.ScopeID, id);
+                UserAccount account = m_Scenes[0].UserAccountService.GetUserAccount(m_Scenes[0].RegionInfo.ScopeID, id);
 
                 if (account != null)
                 {
-                    AddUser (id, account.FirstName, account.LastName);
+                    AddUser(id, account.FirstName, account.LastName);
                 }
                 else
                 {
-                    UserData user = new UserData ();
+                    UserData user = new UserData();
                     user.Id = id;
 
                     if (creatorData != null && creatorData != string.Empty)
                     {
                         //creatorData = <endpoint>;<name>
 
-                        string[] parts = creatorData.Split (';');
+                        string[] parts = creatorData.Split(';');
                         if (parts.Length >= 1)
                         {
-                            user.HomeURL = parts [0];
+                            user.HomeURL = parts[0];
                             try
                             {
-                                Uri uri = new Uri (parts [0]);
+                                Uri uri = new Uri(parts[0]);
                                 user.LastName = "@" + uri.Authority;
                             }
                             catch (UriFormatException)
                             {
-                                m_log.DebugFormat ("[SCENE]: Unable to parse Uri {0}", parts [0]);
+                                m_log.DebugFormat("[SCENE]: Unable to parse Uri {0}", parts[0]);
                                 user.LastName = "@unknown";
                             }
                         }
                         if (parts.Length >= 2)
-                            user.FirstName = parts [1].Replace (' ', '.');
+                            user.FirstName = parts[1].Replace(' ', '.');
                     }
                     else
                     {
@@ -539,7 +543,7 @@ namespace OpenSim.Region.CoreModules.Framework.UserManagement
                         user.LastName = "UserUMMAU";
                     }
 
-                    AddUserInternal (user);
+                    AddUserInternal(user);
                 }
             }
         }