combobox doesn't hold new index on postback if set to 0

Question

I am populating an ajax toolkit combobox based on the selection in another field. On the postback I always want to set the index of the combobox to 0 (the first value in list). If I programmatically always set it to 1 or 2, it works and the selected index and displayed value are correct (assuming there are enough values in the list). However, if I set it to 0, although myCmb.SelectedIndex = 0 at the end of the postback, the selected item is whatever the index was before the postback. This must be a bug, but I can't seem to force it to display the first item in the dropdown. Any ideas?