can't get asp.net ListBox SelectedValue

No matter what I try, I can't get the SelectedValue from my listbox on postback. I have triple-checked and even verified by stepping through code that I'm not rebinding my listbox to the datasource on postback (button click).

listbox declaration:
<asp:ListBox ID="listAvailableAMCs" Rows="8" Width="300px" DataValueField="ID" DataTextField="Company" runat="server"></asp:ListBox>

Listbox data population works fine.

A button click calls this:

protected void LinkAMC(object sender, EventArgs e)
    {
        if(listAvailableAMCs.SelectedValue != string.Empty)
            Utilities.LinkAMC(int.Parse(listAvailableAMCs.SelectedValue));

        LoadGridData();
    }

SelectedValue is always blank. SelectedText is blank and SelectedIndex is -1.

A view source reveals this html:

<select size="8" name="ctl00$MasterMain$listAvailableAMCs" id="MasterMain_listAvailableAMCs" style="width:300px;">
      <option value="19">Como AMC</option>
      <option value="21">Sinatra AMC</option>
      <option value="20">Martin AMC</option>
</select>

Those option values and text values are exactly right.

What am I missing?