restore the PromptText=" " in CascadingDropDown using vb code

I need help;
How can I restore the PromptText="Select Country"  in CascadingDropDown using vb code
i.e, in drop down list I can use ddlCountries.Items.Insert(0, New ListItem("Select Country ", "-1")).
But this code doesn’t work with CascadingDropDown
<asp:DropDownList ID="ddlCountries" runat="server" Width="150px"> </asp:DropDownList> <ajx:CascadingDropDown ID="cdlCountries" TargetControlID="ddlCountries" PromptText="Select Country" PromptValue="" ServicePath="ServiceVB.asmx" ServiceMethod="GetCountries" runat="server" Category="CountryId" LoadingText = "Loading..." />