How do you use a Undo from EditorWindow?

/*

• My CustomEditorWindow -> "Ctrl+Z(Undo)" is not working!!

Unity Version 4.3.4f1

*/

using UnityEditor;

public class SampleWindow : EditorWindow {

 [MenuItem( "Window/Sample Window" )]
 static void ShowWindow()
 {
     EditorWindow.GetWindow( typeof( SampleWindow ), false, "Sample Window" );
 }

 private SampleClass1 src1;

 private void OnEnable()
 {
     src1 = AssetDatabase.LoadAssetAtPath( "Assets/Resources/SampleObj1.prefab", typeof( SampleClass1 ) ) as SampleClass1;
 }

 public void OnGUI()
 {
     var c = src1 as SampleClass1;

     string s = EditorGUILayout.TextField( "Text", c.sampleProperty1 );
     if( c.sampleProperty1 != s )
     {strong text
         // Does'nt work.
         Undo.RecordObject( c, "SampleClass1/String/SampleProperty1/Change" );
         c.sampleProperty1 = s;
     }

     float value = EditorGUILayout.Slider( "Value", c.slider, 0f, 1f );
     if( c.slider != value )
     {
         // Does'nt work.
         Undo.RecordObject( c, "SampleClass1/Float/Slider/Change" );
         c.slider = value;
     }
 }

}