Unable to cast object of type 'System.Web.UI.LiteralControl'

Unable to cast object of type 'System.Web.UI.LiteralControl' error on quantity,amount

protected void gdShoppingCart_RowUpdating(object sender, GridViewUpdateEventArgs e)
{
bool result;

ShopingCartBLL cartObj = new ShopingCartBLL();

TextBox txtId = (TextBox)(gdShoppingCart.Rows[e.RowIndex].Cells[0].Controls[0]);
cartObj.BookId = Convert.ToInt32(txtId.Text);
TextBox txtTitle = (TextBox)(gdShoppingCart.Rows[e.RowIndex].Cells[1].Controls[0]);
cartObj.Title = txtTitle.Text;
TextBox txtPrice = (TextBox)(gdShoppingCart.Rows[e.RowIndex].Cells[2].Controls[0]);
cartObj.Price = Convert.ToDouble(txtPrice.Text);
TextBox txtQuantity = (TextBox)(gdShoppingCart.Rows[e.RowIndex].Cells[3].Controls[0]);
cartObj.Quantity =Convert.ToInt32(txtQuantity.Text);
TextBox txtAmount = (TextBox)(gdShoppingCart.Rows[e.RowIndex].Cells[4].Controls[0]);
cartObj.Amount = Convert.ToDouble(txtAmount.Text);
cartObj.UserId = 2;//replace it with session or pass url
result = cartObj.UpdateAnItem();

if (result)
{
string script = "";
Page.ClientScript.RegisterClientScriptBlock(this.GetType(), "UpDated", script);
gdShoppingCart.EditIndex = -1;
BindData();
}
else
{
string script = "";
Page.ClientScript.RegisterClientScriptBlock(this.GetType(), "UpDated", script);
}

}