错在哪了


<head runat="server">
    <title></title>
    <script type="text/javascript" src="jquery-1.4.2.min.js"></script>
     <script type="text/javascript" language="javascript">
         $("#Button1").click(function () 
         {
             var txtName = $("#<%=txtName.ClientID%>");
             if(txtName.val() == "")
             {
                 alert('111!');
             }
         });
    </script>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:Button ID="Button1" runat="server" Text="Button" />
        <asp:TextBox ID="txtName" runat="server"></asp:TextBox>
    </div>
    </form>
</body>没有错误，点按钮刷新页面，为什么判断不出来