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..." />