那个验证控件可以验证image的Url不能为空解决办法

<form id="form1" runat="server">    <script>        function validate(source, arguments) {            var obj = document.getElementById('<%=Image1.ClientID %>');            var trimLeft = /^\s+/;            var trimRight = /\s+$/;            var src = obj.getAttribute("src");            src = src.replace(trimLeft, "").replace(trimRight, "");            if (src == "") {                arguments.IsValid = false;            } else {                arguments.IsValid = true;            }        }    </script>    <div>        <asp:Image ID="Image1" runat="server" />        <asp:CustomValidator ID="CustomValidator1"            runat="server" ErrorMessage="CustomValidator"             ClientValidationFunction="validate"></asp:CustomValidator>        <asp:Button ID="Button1" runat="server" Text="Button" onclick="Button1_Click" />    </div>    </form>