ASP.NET表单验证第一次不起作用

<asp:UpdatePanel ID="LoginPanel" UpdateMode="Conditional" runat="server">
    <ContentTemplate>
        <div id="login">
            <div class="row">
                <div class="label">
                    <asp:Label ID="lblUsername" Text="<%$Resources:Login,UserNameField %>" runat="server" />
                </div>
                <div class="field">
                    <asp:TextBox ID="txtUsername" MaxLength="12" runat="server" />
                    <asp:RequiredFieldValidator ID="rfvUsername" ControlToValidate="txtUsername" ValidationGroup="vgLogin" SetFocusOnError="true"
                        ErrorMessage="*" ToolTip="<%$Resources:Login,UserNameRequired %>" runat="server" />
                </div>
            </div>

            <div class="row">
                <div class="label">
                    <asp:Label ID="lblPassword" Text="<%$Resources:Login,PasswordField %>" runat="server" />
                </div>
                <div class="field">
                    <asp:TextBox ID="txtPassword" MaxLength="12" TextMode="Password" runat="server" />
                    <asp:RequiredFieldValidator ID="rfvPassword" ControlToValidate="txtPassword" ValidationGroup="vgLogin" SetFocusOnError="true"
                        ErrorMessage="*" ToolTip="<%$Resources:Login,PasswordRequired %>"  runat="server" />
                </div>
            </div>

            <div class="row">
                <div class="label">
                    <asp:Label ID="lblRemember" Text="<%$Resources:Login,RememberField %>" runat="server" />
                </div>
                <div>
                    <asp:CheckBox ID="chkRemember" Checked="true" ToolTip="<%$Resources:Login,RememberToolTip %>" runat="server" />
                </div>
            </div>

            <div class="buttons">
                <asp:Button ID="btnLogin" Text="<%$Resources:Login,Command %>" OnClick="btnLogin_Click" ValidationGroup="vgLogin" CausesValidation="true" runat="server" />
            </div>
        </div>
    </ContentTemplate>
</asp:UpdatePanel>

第一次,验证器不会检查字段是否已完成,无论如何都会提交表单,在初始打嗝之后,表单每次都会正确验证.

我知道我可以问(并且应该,无论如何)在服务器端是否有Page.IsValid,但我仍然希望验证能够在第一次正确警告用户输入错误,而不是先等待服务器响应.

我究竟做错了什么？