在VS2005Bate1中，GridView修改数据后不能保存

在VS2005Bate1中，GridView修改数据后不能保存，提示：必须声明变量 '@ProductID'
具体如下：
<html>
<head id="Head1" runat="server">
    <title>Edit GridView</title>
</head>
<body>
    <form id="Form1" runat="server">
    <asp:GridView ID="GridView1"  
        DataSourceID="ProductsSource"
        AutoGenerateEditButton="true"
        DataKeyNames="ProductID"
        Runat="Server" />
    
    <asp:SqlDataSource 
        ID="ProductsSource"
        ConnectionString=
          "Server=localhost;Database=Northwind;Trusted_Connection=true"
        SelectCommand=
          "SELECT ProductID,ProductName,Discontinued FROM Products"
        UpdateCommand="Update Products 
            SET ProductName=@ProductName,Discontinued=@Discontinued 
            WHERE ProductID=@ProductID"
            Runat="Server" />
    </form>
</body>
</html>