Validation not stopping update

I have a ListView with UpdatePanel around it.  There is a TextBox control with a CompareValidator to make sure an entry in the TextBox does not exceed the value in another. The validator error message shows in red but the page still posts back the update and I don't know why.  Below is the markup for TextBox and Validator.
<asp:TextBox ID="txdailypro" runat="server" Text='<%# Bind("DailyPro") %>' Width="60" OnTextChanged="ProteinChanged" ClientIDMode="Static" BackColor="#FFFF99" AutoPostBack="True" /> <asp:CompareValidator ID="CompareValidator1" runat="server" ErrorMessage="Cannot exceed total Protein+Carb" Type="Double" ...