ListBox.Selected value doesnt show any value

I have a ListBox control "UnSelectedTagsList"
I have ImageButton control "FullyRemoveTag".
On the FullyRemoveTag_Click event, the UnSelectedTagsList.SelectedValue gives empty value even there is item selected.
Where is the problem?
I observed that when I use button instead of imagebutton, I can get the UnSelectedTagsList.SelectedValue.
Why there is problem in getting the ListBox.selectedvalue when using ImageButton?
Regards,