ASP.NET UpdatePanel+回发无效

我有以下非常简单的形式:

<asp:UpdatePanel ID="ClaimRewardsForm" runat="server">
    <ContentTemplate>
        <span class="largeBold">Select jacket weight:</span><br />
        <asp:RadioButtonList runat="server" ID="JacketWeight">
            <asp:ListItem Value="Lightweight" Text="Lightweight (fleece)" />
            <asp:ListItem value="Heavyweight" Text="Heavyweight (cotton)" />                                
        </asp:RadioButtonList>
        <br />
        <span class="largeBold">Select size:</span><br />
        (Men's sizes only)<br />
        <asp:DropDownList ID="JacketSize" runat="server">
            <asp:ListItem Value="Small" Text="Small" />
            <asp:ListItem Value="Medium" Text="Medium" />
            <asp:ListItem Value="Large" Text="Large" />
        </asp:DropDownList><br />
        <br />
        <asp:ImageButton ID="SubmitButton" runat="server" ImageUrl = "~/Content/Images/submitButton.png" onclick="SubmitButton_Click" />
    </ContentTemplate>
</asp:UpdatePanel>       

在我的按钮的单击处理程序中,我有:

protected void SubmitButton_Click(object sender, EventArgs e)
{
    if (IsValid)
    {
        using (var work = UnitOfWorkFactory.Create())
        {
            var id = new Guid(Session["id"].ToString());
            var account = UserAccounts.Get(id);

            if (account == null)
                throw new Exception("Invalid user account id.");

            account.RewardInfo.Clear();
            account.RewardInfo.Add(new RewardInfo()
            {
                UserAccount = account,
                JacketWeight = JacketWeight.SelectedValue,
                JacketSize = JacketSize.SelectedValue
            });

            work.Commit();
        }

        //ClaimRewardsForm.Update();
        //ScriptManager.RegisterStartupScript(this, GetType(),
        //    "confirmation", "ClaimRewards.showConfirmation();", true);
    }
}

我不会以任何方式修改表单字段,但仍然出现以下错误:

505|error|500|Invalid postback or callback argument.  Event validation is enabled using <pages enableEventValidation="true"/> in configuration or <%@ Page EnableEventValidation="true" %> in a page.  For security purposes, this feature verifies that arguments to postback or callback events originate from the server control that originally rendered them.  If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method in order to register the postback or callback data for validation.|

因为我在发帖过程中根本没有修改任何控件,所以我一辈子都无法弄明白为什么它会像我一样。有什么想法吗?