Dropdown menu with XML data source gives web part error

Question

I am using SharePoint designer to build a page with a data view web part that displays the contents of a 6,000 record XML file that is structured like the example below with each "Details" element being a record.

<?xml version="1.0" encoding="UTF-8"?>
<Details_Collection>
   <Details>
      <Display_Name>Weather, Wendy</Display_Name>
      <Age>27</Age>
      <Gender>Female</Gender>
      <Marital_Status>Married</Marital_Status>
      <Email>person@test.edu</Email>
      <Site_Location>Denver, CO</Site_Location>
   </Details>
   <Details>
       ...
   </Details>
</Details_Collection>

I want to filter that DVWP by the Site_Location element using an ASP dropdown control. Rather than have to maintain a list of about a hundred Site Locations as choices in the dropdown control's code, I want to simply use that XML file as the data source for my dropdown's choices.

I can insert the XML data source onto the page just fine. But, when I select it as the data source for my dropdown and then preview the page, it gives me a web part error and the page won't load.

I assume that what I am doing is possible. Is it not working because my XML file doesn't have any schema information? It seems to be well-formed enough to load in the DVWP. After-all, the entire page works great UNTIL I associate the ASP dropdown with the data source. That's when it breaks.