Oncheckedchange is not working in my page.... But selectedindex change is firing ...I am using master page also

<%@ Page Title="" Language="C#" MasterPageFile="~/Main.master" AutoEventWireup="true" CodeFile="testingt.aspx.cs" Inherits="testingt" EnableEventValidation="true" %>

<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
   
     <asp:CheckBox ID="CheckBox1" Text="I agree" runat="server" OnCheckedChanged="CheckBox1_CheckedChanged1" AutoPostBack="true"  ViewStateMode="Enabled" EnableViewState="true" />
     <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
       <div class="col-md-6">
        <div class="form-group">
         <asp:DropDownList ID="ddlSenderID" runat="server" Enabled="true"  class="form-control" OnSelectedIndexChanged="ddlSenderID_SelectedIndexChanged" AutoPostBack="true" EnableViewState="true" ViewStateMode="Disabled">
               <asp:ListItem Text="SENDER ID" Value="0"></asp:ListItem>
                <asp:ListItem Text="HADIYA" Value=""></asp:ListItem>
                   </asp:DropDownList>
                   </div>
                    </div>
</asp:Content>

 protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
        }
     }
         protected void ddlSenderID_SelectedIndexChanged(object sender, EventArgs e)
         {
         }

         protected void CheckBox1_CheckedChanged1(object sender, EventArgs e)
         { 
             TextBox1.Text = "testingg"; 
         }
}