diff --git a/OpenSim/Region/Framework/Scenes/SceneObjectPart.cs b/OpenSim/Region/Framework/Scenes/SceneObjectPart.cs
index 6518b84e49..6677daeb30 100644
--- a/OpenSim/Region/Framework/Scenes/SceneObjectPart.cs
+++ b/OpenSim/Region/Framework/Scenes/SceneObjectPart.cs
@@ -4556,10 +4556,20 @@ namespace OpenSim.Region.Framework.Scenes
         /// Get a copy of the list of sitting avatars.
         /// </summary>
         /// <remarks>This applies to all sitting avatars whether there is a sit target set or not.</remarks>
-        /// <returns></returns>
+        /// <returns>A hashset of the sitting avatars.  Returns null if there are no sitting avatars.</returns>
         public HashSet<UUID> GetSittingAvatars()
         {
-            return new HashSet<UUID>(m_sittingAvatars);
+            HashSet<UUID> sittingAvatars = m_sittingAvatars;
+
+            if (sittingAvatars == null)
+            {
+                return null;
+            }
+            else
+            {
+                lock (sittingAvatars)
+                    return new HashSet<UUID>(sittingAvatars);
+            }
         }
 
         /// <summary>
diff --git a/OpenSim/Region/Framework/Scenes/Tests/ScenePresenceSitTests.cs b/OpenSim/Region/Framework/Scenes/Tests/ScenePresenceSitTests.cs
index ed39be1fec..493ab70883 100644
--- a/OpenSim/Region/Framework/Scenes/Tests/ScenePresenceSitTests.cs
+++ b/OpenSim/Region/Framework/Scenes/Tests/ScenePresenceSitTests.cs
@@ -26,6 +26,7 @@
  */
 
 using System;
+using System.Collections.Generic;
 using System.Reflection;
 using Nini.Config;
 using NUnit.Framework;
@@ -69,6 +70,8 @@ namespace OpenSim.Region.Framework.Scenes.Tests
             m_sp.HandleAgentRequestSit(m_sp.ControllingClient, m_sp.UUID, part.UUID, Vector3.Zero);
 
             Assert.That(part.SitTargetAvatar, Is.EqualTo(UUID.Zero));
+            Assert.That(part.GetSittingAvatarsCount(), Is.EqualTo(0));
+            Assert.That(part.GetSittingAvatars(), Is.Null);
             Assert.That(m_sp.ParentID, Is.EqualTo(0));
         }
 
@@ -86,7 +89,13 @@ namespace OpenSim.Region.Framework.Scenes.Tests
 
             m_sp.HandleAgentRequestSit(m_sp.ControllingClient, m_sp.UUID, part.UUID, Vector3.Zero);
 
+            Assert.That(m_sp.PhysicsActor, Is.Null);
+
             Assert.That(part.SitTargetAvatar, Is.EqualTo(UUID.Zero));
+            Assert.That(part.GetSittingAvatarsCount(), Is.EqualTo(1));
+            HashSet<UUID> sittingAvatars = part.GetSittingAvatars();
+            Assert.That(sittingAvatars.Count, Is.EqualTo(1));
+            Assert.That(sittingAvatars.Contains(m_sp.UUID));
             Assert.That(m_sp.ParentID, Is.EqualTo(part.LocalId));
         }
 
@@ -104,10 +113,6 @@ namespace OpenSim.Region.Framework.Scenes.Tests
 
             m_sp.HandleAgentRequestSit(m_sp.ControllingClient, m_sp.UUID, part.UUID, Vector3.Zero);
 
-            Assert.That(part.SitTargetAvatar, Is.EqualTo(UUID.Zero));
-            Assert.That(m_sp.ParentID, Is.EqualTo(part.LocalId));
-            Assert.That(m_sp.PhysicsActor, Is.Null);
-
             // FIXME: This is different for live avatars - z position is adjusted.  This is half the height of the
             // default avatar.
             // Curiously, Vector3.ToString() will not display the last two places of the float.  For example,
@@ -119,6 +124,8 @@ namespace OpenSim.Region.Framework.Scenes.Tests
             m_sp.StandUp();
 
             Assert.That(part.SitTargetAvatar, Is.EqualTo(UUID.Zero));
+            Assert.That(part.GetSittingAvatarsCount(), Is.EqualTo(0));
+            Assert.That(part.GetSittingAvatars(), Is.Null);
             Assert.That(m_sp.ParentID, Is.EqualTo(0));
             Assert.That(m_sp.PhysicsActor, Is.Not.Null);
         }
@@ -145,11 +152,20 @@ namespace OpenSim.Region.Framework.Scenes.Tests
                 Is.EqualTo(part.AbsolutePosition + part.SitTargetPosition + ScenePresence.SIT_TARGET_ADJUSTMENT));
             Assert.That(m_sp.PhysicsActor, Is.Null);
 
+            Assert.That(part.GetSittingAvatarsCount(), Is.EqualTo(1));
+            HashSet<UUID> sittingAvatars = part.GetSittingAvatars();
+            Assert.That(sittingAvatars.Count, Is.EqualTo(1));
+            Assert.That(sittingAvatars.Contains(m_sp.UUID));
+
             m_sp.StandUp();
 
             Assert.That(part.SitTargetAvatar, Is.EqualTo(UUID.Zero));
             Assert.That(m_sp.ParentID, Is.EqualTo(0));
             Assert.That(m_sp.PhysicsActor, Is.Not.Null);
+
+            Assert.That(part.SitTargetAvatar, Is.EqualTo(UUID.Zero));
+            Assert.That(part.GetSittingAvatarsCount(), Is.EqualTo(0));
+            Assert.That(part.GetSittingAvatars(), Is.Null);
         }
 
         [Test]
diff --git a/prebuild.xml b/prebuild.xml
index 45f58c7e7c..3419119ef6 100644
--- a/prebuild.xml
+++ b/prebuild.xml
@@ -3042,6 +3042,7 @@
 
       <ReferencePath>../../../bin/</ReferencePath>
       <Reference name="System"/>
+      <Reference name="System.Core"/>
       <Reference name="System.Xml"/>
       <Reference name="System.Drawing"/>
       <Reference name="System.Runtime.Remoting"/>