Error Updating Dynamic Datagrid With SQL Server

Hello, I have a datagrid which is populated with data from an MS SQL server database.
 When I run an update query it always throws an exception - what is the most likely cause for this given that I am using the code below:
 
 1 public void DataGrid_Update(Object sender, DataGridCommandEventArgs e)
2 {
3 String update = "UPDATE Fruit SET Product = @ID, Quantity = @Q, Price = @P, Total = @T where Product = @Id";
4
5 SqlCommand command = new SqlCommand(update, conn);
6
7 command.Parameters.Add(new SqlParameter("@ID", SqlDbType.NVarChar, 50));
8 command.Parameters.Add(new SqlParameter("@Q", SqlDbType.NVarChar, 50));
9 command.Parameters.Add(new SqlParameter("@P", SqlDbType.NVarChar, 50));
10 command.Parameters.Add(new SqlParameter("@T", SqlDbType.NVarChar, 50));
11 command.Parameters["@ID"].Value = DataGrid.DataKeys[(int)e.Item.ItemIndex];
12 command.Connection.Open();
13
14 try
15 {
16 command.ExecuteNonQuery();
17 Message.InnerHtml = "Update complete!" + update;
18 DataGrid.EditItemIndex = -1;
19 }
20 catch (SqlException exc)
21 {
22 Message.InnerHtml = "Update error.";
23 }
24
25 command.Connection.Close();
26
27 BindGrid();
28 }

 
All of the row types in MS SQL server are set to nvarchar(50) - as I thought this would eliminate any inconsistencies in types. 
Thanks anyone