Writing data from dropdown menues to table

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C#

ASP.NET

hi guys.
bit of a problem i have come across on my final page, i am taking information which are integers from drop down menus which are, in turn linked to their own data sources.
and then writing these intergers to a different table which are the same types (int) as the tables they are coming from, even then i am changing them to strings during the parameter:

<pre lang="cs">protected void confirmFixedHolButton0_Click(objectsender, EventArgs e)
        {
            using (conn)
            {
                conn.Open();
                SqlCommand insertHolBooking = new SqlCommand("INSERT INTO [HolidayBooking] ([Customer ID], [Fixed Holiday ID], [Holiday ID], [Flight ID], [Holiday Tour ID])
 VALUES 
(@customerID, @fixHolID, @holID, @flightID, @holTourID)", conn);
                insertHolBooking.CommandType = CommandType.Text;

                insertHolBooking.Parameters.AddWithValue("@customerID", customerIDDDM.SelectedItem.ToString());
                insertHolBooking.Parameters.AddWithValue("@fixHolID", fixedHolDDL.SelectedItem.ToString());
                insertHolBooking.Parameters.AddWithValue("@holID", holidayIDDDM.SelectedItem.ToString());
                insertHolBooking.Parameters.AddWithValue("@flightID", flightIDDDM.SelectedItem.ToString());
                insertHolBooking.Parameters.AddWithValue("@holTourID", tourIDDDM0.SelectedItem.ToString());

                try
                {   conn.Open();
                    insertHolBooking.ExecuteNonQuery();
                    Response.Redirect("Bookings.aspx");
                }
                catch (Exception)
                {   Response.Write("<b>Failed please try again.</b>");
                }
            }
        }

it always throws the error message, cant see anything wrong with the SQL nor with the parameter.

The table this information is being written to is allowing nulls so i cant see the problem...

please help guys
Regards
Matt.