Server Error in '/' Application. 
 
'lstQuantity' has a SelectedValue which is invalid because it does not exist in the list of items.
Parameter name: value 
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.ArgumentOutOfRangeException: 'lstQuantity' has a SelectedValue which is invalid because it does not exist in the list of items.
Parameter name: value

<EditItemTemplate>
        <asp:DropDownList ID="lstQuantity" runat="server" SelectedValue='<%# Eval("Quantity") %>'
          AutoPostBack="True" OnSelectedIndexChanged="lstQuantity_SelectedIndexChanged">
          <asp:ListItem Value="1" Selected="True">1</asp:ListItem>
          <asp:ListItem Value="2">2</asp:ListItem>
          <asp:ListItem Value="3">3</asp:ListItem>
          <asp:ListItem Value="4">4</asp:ListItem>
          <asp:ListItem Value="5">5</asp:ListItem>
          <asp:ListItem Value="6">6</asp:ListItem>
          <asp:ListItem Value="7">7</asp:ListItem>
          <asp:ListItem Value="8">8</asp:ListItem>
          <asp:ListItem Value="9">9</asp:ListItem>
          <asp:ListItem Value="10">10</asp:ListItem>
        </asp:DropDownList>
      </EditItemTemplate>

public partial class controls_ShoppingCartView : System.Web.UI.UserControl
	{
	    
	    private bool _isReadOnly = false;
	    decimal priceTotal = 0;
	    int quantityTotal = 0;
	    
	    public event EventHandler CartUpdated;
	    
	    /// <summary>
	    /// Fires when a row in the GridView is updated. Triggers an event that can be handled by a page containing this User Control.
	    /// </summary>
	    protected void GridView1_RowUpdated(object sender, System.Web.UI.WebControls.GridViewUpdatedEventArgs e)
	    {
	        
	        if (CartUpdated != null) {
	            CartUpdated(sender, new System.EventArgs());
	        }
	    }
	    
	    /// <summary>
	    /// Fires when a row in the GridView is deleted. Triggers an event that can be handled by a page containing this User Control.
	    /// </summary>
	    protected void GridView1_RowDeleted(object sender, System.Web.UI.WebControls.GridViewDeletedEventArgs e)
	    {
	        if (CartUpdated != null) {
	            CartUpdated(sender, new System.EventArgs());
	        }
	    }
	    
	    public bool ReadOnly {
	        get { return _isReadOnly; }
	        set { _isReadOnly = value; }
	    }
	    
	    protected void Page_Load(object sender, System.EventArgs e)
	    {
	        if (_isReadOnly) {
	            GridView1.Columns[5].Visible = false;
	            GridView1.Columns[6].Visible = false;
	        }
	    }
	    
	    protected void GridView1_RowDataBound(object sender, System.Web.UI.WebControls.GridViewRowEventArgs e)
	    {
	        switch (e.Row.RowType) {
	            case DataControlRowType.DataRow:
	                // For each item in the datasource, get the quantity and subtotal, and store them in
	                // private variables called priceTotal and quantityTotal. When the Footer is databound,
	                // these totals are added to the Footer.
	                priceTotal += Convert.ToDecimal(DataBinder.Eval(e.Row.DataItem, "SubTotal"));
	                quantityTotal += Convert.ToInt32(DataBinder.Eval(e.Row.DataItem, "Quantity"));
	                break;
	            case DataControlRowType.Footer:
	                e.Row.Cells[0].Text = "Total:";
	                // Display the item sub total and order total in the footer
	                e.Row.Cells[2].Text = quantityTotal.ToString();
	                e.Row.Cells[4].Text = priceTotal.ToString("c");
	                
	                e.Row.Cells[2].HorizontalAlign = HorizontalAlign.Right;
	                e.Row.Cells[4].HorizontalAlign = HorizontalAlign.Right;
	                break;
	        }
	    }
	    
	    protected void lstQuantity_SelectedIndexChanged(object sender, System.EventArgs e)
	    {
	        GridView1.UpdateRow(GridView1.EditIndex, false);
	        GridView1.EditIndex = -1;
	    }
	    
	    protected void odsShoppingCart_Updating(object sender, System.Web.UI.WebControls.ObjectDataSourceMethodEventArgs e)
	    {
	        // Get the Id and the new quantity from the form and assign them to the InputParams for the Update method.
	        // The two params will be passed to the updated method in the UpdateProductInCart method of the ShopManager.
	        e.InputParameters["Id"] = new Guid(GridView1.DataKeys[GridView1.EditIndex].Value.ToString());
	        e.InputParameters["newQuantity"] = Convert.ToInt32(((DropDownList)GridView1.Rows[GridView1.EditIndex].FindControl("lstQuantity")).SelectedValue);
	    }
	}