Default button not working in asp.net panel

I have an asp.net panel with a default button setting but the button is never trigger when hitting the ENTER button. I searched SO for this question, but could not find any response that works.

How can I get btnRegister to click when the users hits ENTER ?

<asp:Panel runat="server" ID="pnlCustomer" DefaultButton="btnRegister">
    <table width="500px" border="0" cellpadding="0" cellspacing="0">
    <tr>
        <td align="right">
            <asp:Button ID="btnBackToProfile" Text="Back To Profile" runat="server" Enabled="false"
                onclick="btnBackToProfile_Click" />
        </td>
        <td align="right">
            <asp:Button ID="btnClearnForm" Text="Clear Form" runat="server" 
                OnClientClick="ClearForm1()" />
        </td>
        <td align="right">
            <asp:Button id="btnRegister" runat="server" onclientclick="document.getElementById('AlertTime').value = GetSeconds();" Text="Continue" OnClick="btnRegister_Click" />
        </td>
    </tr>
    </table>
</asp:Panel>