Texture does not appear when applied on the thumb and slider of vertical slider

Hi all,

There is some problem when I was coding the texture onto the vertical slider. The texture does not appear and also the default thumb for the vertical slider also missing. But once i remove the texture, everything back to normal.

Here is my code:

 var volumeSlider : GUIStyle;
 var volumeThumb : GUIStyle;
 if (optionMenu) {
      GUI.Box (Rect (0,0,960,640), "Option", optionBg);
      musicSlider = GUI.VerticalSlider (Rect (185,180,12,216), musicSlider, 10.0, 0.0, , volumeSlider, volumeThumb);
     AudioListener.volume = musicSlider/10.0;
     if (!display) {
         display = true;
     } else if (display) {
         display = false;
     }
     if (!display) {
         
         if (GUI.Button (Rect (100,550,128,64), "Back", backBtn)) {
             mainMenu = true;
             optionMenu = false;
          }
     }
     menu();
      return;
  }

How to solve this problem?