为什么asp.net找不到我的文本框?

我正在尝试向 CreateUserWizardStep ,我补充说:

<asp:CreateUserWizardStep ID="CreateUserWizardStep1" runat="server">
    <ContentTemplate>
        <table border="0">
            <tr>
                <td align="right">
                    <asp:Label ID="NickNameLabel" runat="server" AssociatedControlID="NickName">Nick Name:</asp:Label>
                </td>
                <td>
                    <asp:TextBox ID="NickName" runat="server"></asp:TextBox>
                    <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="NickName"
                        ErrorMessage="Nick Name is required." ToolTip="Nick Name is required." ValidationGroup="CreateUserWizard1">*</asp:RequiredFieldValidator>
                </td>
            </tr>
            <%-- The default code is left unchanged, but not shown here --%>
        </table>
    </ContentTemplate>
</asp:CreateUserWizardStep>

然后我试着引用这样的对象

protected void NewUserWizard_CreatedUser(object sender, EventArgs e)
{
    CreateUserWizardStep step = NewUserWizard.FindControl("CreateUserWizardStep1") as CreateUserWizardStep;
    TextBox nickName = step.FindControl("NickName") as TextBox;
    // insert additional information to the database
}

问题是,我得到的是空值 nickName FindControl("") 不正确?