"Objects has been deleted." Error when tried to add smart tag on drop-down list content control

Hi,

I wanted to add smart tag in content control in word 2007 (I am doing it on content Control exit event). I have written following code in content control exit event to achieve the same. This works fine when Rich Text Content Control is used but the moment i use drop-down list content control; it throws an exception saying "Objects has been deleted."

I watch value of osmartTag object in QuickWatch, and found that most of object threw an exception of type 'System.Runtime.InteropServices.COMException'

Object prope1 = null;

Object range1 = (Word.Range)ContentControl.Range;

Word.SmartTag osmartTag = ContentControl.Range.SmartTags.Add("http://schemas.openxmlformats.org/1 2006/smarttags#TeamCenter",ref range1, ref prope1);