how to use require field validator for drop down which was bind from sql data set . i am not able to get the error message when i am clicking submit button

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C#

ASP.NET

I bind drop down with Data set like this:

 ddlVendorname.DataSource = ds.Tables["Table1"];
ddlVendorname.DataTextField = "VENDOR_NAME";
ddlVendorname.DataValueField = "VENDOR_ID";
ddlVendorname.DataBind();
ddlVendorname.Items.Insert(0, "--Select--");

And My Aspx With Required Fields Validator:

<asp:DropDownList ID="ddlVendorname" runat="server" CssClass="dropdown185" AutoPostBack="True" OnSelectedIndexChanged="ddlVendorname_SelectedIndexChanged" AppendDataBoundItems="true">

                                
 <asp:RequiredFieldValidator ID="ReqVendorName" runat="server" ErrorMessage="Please Select Vendor Name" ControlToValidate="ddlVendorname" InitialValue="0" >

But I am unable to get the error message when i am pressing the submit Button

<asp:ImageButton ID="imgbtnSave" runat="server" ImageUrl="~/Buttons/save-btn.png"
                                               AlternateText="Save" OnClick="imgbtnSave_Click" CausesValidation="true" />

PZ Some one help me to solve this...