Error - 'Style' is null or not an object

I have just upgraded to version 2.6.6.

Now whenever the editor is focused and I click on the submit button a error occurs in RefreshActiveItems method of fcktoolbarstylecombo.

for ( var i in targetSpecialCombo.Items )
{
   var item = targetSpecialCombo.Items[i] ;
   var style = item.Style ;

   if ( style.CheckElementRemovable( elements[ e ], true ) )
   {
      targetSpecialCombo.SetLabel( style.Label || style.Name ) ;
      return ;
   }
}

The item.Style is the problem.
I thought of some error in my fckstyles.xml file, but it does not work with the original fckstyles.xml either.

Thanks for help.
Thomas

PS: I am using IE8.