Simple ListBox Validator

I ran into a situation today where I was copying values from a TextBox to a ListBox and before submitting the form, I needed to validate that there were actually values in the ListBox. Sounds simple enough, right? At first I tried using a RequiredFieldValidator and it seemed to work. But, I was able to get the form into a state where it was incorrectly flagging the ListBox value as invalid. Here is a simple ListBox validator that I wrote for this situation.

using System.Web.UI.WebControls;
public class ListBoxRequiredFieldValidator : CustomValidator
{
    public ListBoxRequiredFieldValidator()
    {
        EnableClientScript = true;
        ValidateEmptyText = true;
    }

    protected override void OnPreRender(EventArgs e)
    {
        ClientValidationFunction = 
                string.Concat(ClientID, "_ValidateRequiredListBox");
        base.OnPreRender(e);
        string validatedcontrolclientid = 
                GetControlRenderID(ControlToValidate);
        const string scriptFormat =
            @"";

        var script = string.Format(scriptFormat,
                                    ClientID,
                                    validatedcontrolclientid);
        Page.ClientScript.RegisterClientScriptBlock(this.GetType(), 
                string.Concat(ClientID, "_ValidationScript"), script);
    }
}