Form not posting by Enter button

Hi All,
I am trying to associate the Enter/Return key to a form Search button. When I enter some data in input boxes and press enter it does not post the form even though I have declared default button in aspx page. My code is:
<asp:Panel ID="panelSearch" DefaultButton="Search" runat="server" >   <asp:Button runat="server" ID="cmdSearch" Text="Search"  ValidationGroup="ValidateId" onclick="cmdSearch_Click" />
 </asp:Panel>
 
Can anyone please tell me what I am doing wrong?
Thanks.
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