Column quantity does not belong to table cart

Hi
I am creating a cart and have come to a halt. I have created a datatable called 'CART' with columns id, quantity, prodcut and cost.
But when i press add button it adds a rows in the datatable but i get the message 'Column quantity does not belong to table cart'
Please could some one assist. COde is below


public void makeCart()
{
DataColumn cartCol = new DataColumn();

cartCol = cartDT.Columns.Add("ID", typeof(Int32));

cartCol.AutoIncrement = true;
cartCol.AutoIncrementSeed = 1;


DataColumn colQuantity = new DataColumn();
colQuantity = new DataColumn();
colQuantity.DataType = System.Type.GetType("System.String");
colQuantity.ColumnName = "Quantity";
colQuantity.DefaultValue = "Quantity";
ca ...