RequiredFieldValidator disappears after partial postback

Hi,
I've coded myself into a state of confusion. I have a form with a postal code TextBox and a country DropDownList both inside an UpdatePanel. When the selected country is changed, the DropDownList auto posts back and sets the regular expression used for validating the postal code. The country DropDownList also has a RequiredFieldValidator. The first item in my DropDownList is empty (Value = Text = String.Empty) and when I select this item, I momentarily see the RequiredFieldValidator appear and then disappear. It seems that the client side script is displaying the error message, but then after my partial postback it disappears. I can see in the country DropDownList's SelectedIndexChanged event handler that the RequiredFieldValidator has IsValid = false, so why doesn't the error message stay visible? Thanks!
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