title: DropDownList "has a SelectedValue which is invalid because it does not exist in the list of items"
body: <p>I have a ASP.NET DropDownList, which I data-bind to a list of objects, and then it throws an exception. This error is driving me nuts, because the message is wrong, and the value clearly <em>does</em> exist in the list of items, as shown by the screenshot:
  <img src="https://i.stack.imgur.com/VgHPh.png" alt="enter image description here"></p>
  
  <p>Here's the relevant ASP.NET markup:</p>
  
  <pre><code>&lt;asp:DropDownList ID="_ddlRequests" runat="server" AppendDataBoundItems="true" DataTextField="RequestNumber" DataValueField="RequestNumber"&gt;
      &lt;asp:ListItem Text="" Value="" /&gt;
  &lt;/asp:DropDownList&gt;
  </code></pre>