asp:DropDownList提交选择输入页面

我有一个asp.net页面,它有一个带有默认按钮的面板,更新面板和更新面板内的dropdownlist。当我用向上/向下键选择一个项目并按回车键选择该项目时填写表单时,表单正在提交。我该怎么解决?

编辑 一些代码:

<asp:SqlDataSource ID="sdsCountries" runat="server" ConnectionString="cs"
SelectCommand="SELECT CountryID, Title FROM Countries"/>

<asp:DropDownList ID="ddlCountries" runat="server" DataSourceID="sdsCountries"
DataValueField="CountryID" DataTextField="Title" AutoPostBack="True"
OnSelectedIndexChanged="ddlCountries_SelectedIndexChanged"/>

<asp:UpdatePanel ID="upState" runat="server" UpdateMode="Conditional">
    <ContentTemplate>
        <asp:SqlDataSource ID="sdsStates" runat="server" ConnectionString="cs"
    SelectCommand="SELECT StateID, Title FROM States WHERE CountryID=@CountryID">
            <SelectParameters><asp:ControlParameter Name="CountryID"
         ControlID="ddlCountries" PropertyName="SelectedValue" DefaultValue="1"/>
            </SelectParameters>
        </asp:SqlDataSource>
        <asp:DropDownList ID="ddlStates" runat="server" DataSourceID="sdsStates"
        DataValueField="StateID" DataTextField="Title"/>
    </ContentTemplate>
    <Triggers>
        <asp:AsyncPostBackTrigger ControlID="ddlCountries"
        EventName="SelectedIndexChanged" />
    </Triggers>
</asp:UpdatePanel>