C# - Panel DefaultButton And GridView Control With EditItemTemplate?

I have a GridView control on a page with a Panel that has a DefaultButton. I am trying to get the "Update" button to work with the Enter key when a user edits a row. It works fine with the mouse. When I click on "Update", the row reverts back to View mode and the updated value is shown.

When the GridView is in the Panel, the Panel's default button (which submits the page) fires when I press Enter, before the RowCommand is even handled, and the update is lost.

When the GridView is not in the Panel, some other seemingly random button in the Panel fires, but not the "Update" button in my row's EditItemTemplate. Specifically, there is a CalendarExtender on the page, and the extender's popup button fires. Totally not what I want.

How can I get this to behave correctly?

Here is the structure of my mark-up code:

<asp:Panel ID="Panel1" runat="server" DefaultButton="btnSubmit">
<!--Form with controls-->
<asp:ImageButton ID="btnWSPODateCal" runat="server" />
<!--this button fires when I press enter while editing a grid row:-->
<ajaxToolkit:CalendarExtender ID="CalendarExtender1" runat="server"
TargetControlID="tbPODate" PopupButtonID="btnWSPODateCal" />
<!--more controls-->
<div class="button_row">
<asp:ImageButton ID="btnCancel" runat="server" />
<asp:ImageButton ID="btnSubmit" OnClick="btnSubmit_Click" />
</div>
</asp:Panel>
<asp:GridView runat="server">
<Columns>
<asp:TemplateField>
<EditItemTemplate>
<asp:ImageButton CommandName="MyUpdate"
ID="btnSubmitRow" runat="server"
CommandArgument="<%# ((GridViewRow) Container).DataItemIndex %>" />