Cast from string to type Double not valid

I've got a drop down list on an asp.net form and I'm inserting the
selected text to a an SQL Server database but i'm encountering the
error 'Cast from string "On Hold" to type 'Double' is not valid.'

This is the code:

 Sub btnSubmit_Click(Sender As Object, E As EventArgs)
 'If Not IsPostBack Then
 If Page.IsValid Then

               Dim strSQL As String

               strSQL = "UPDATE tbl_THA SET " & _
                               "Line_No = @Line, " & _
                               "Zone = @Zone, " & _
                               "Product = @Product, " & _
                               "Process_Fail_Mode = @PFM, " & _
                               "CPM = @CPM, " & _
                               "Lot_Id = @LotID, " & _
                               "No_of_Reels_on_Hold = @NumReels, " & _
                               "Qty_On_Hold = @Qty, " & _
                               "Reel_Id = @ReelID, " & _
                               "Fallout = @Fallout, " & _
                               "Status = @Status, " & _
                               "Operations_Comments = @Comments " & _
                               "WHERE THA_on_hold_No = "&txtTHA_No.Text

               Dim strConnection As String =
ConfigurationSettings.AppSettings("msle02")
               Dim objConnection As New SqlConnection(strConnection)

               Dim dbComm As New SqlCommand(strSQL, objConnection)

               dbComm.Parameters.Add("@Line", SqlDbType.NVarChar, 50)
               dbComm.Parameters.Add("@Zone", SqlDbType.NVarChar, 50)
               dbComm.Parameters.Add("@Product", SqlDbType.NVarChar, 50)
               dbComm.Parameters.Add("@Shift", SqlDbType.NVarChar, 50)
               dbComm.Parameters.Add("@PFM", SqlDbType.NVarChar, 50)
               dbComm.Parameters.Add("@Originator", SqlDbType.NVarChar, 50)
               dbComm.Parameters.Add("@CPM", SqlDbType.NVarChar, 50)
               dbComm.Parameters.Add("@LotID", SqlDbType.NVarChar, 50)
               dbComm.Parameters.Add("@NumReels", SqlDbType.int, 4)
               dbComm.Parameters.Add("@Qty", SqlDbType.int, 4)
               dbComm.Parameters.Add("@ReelID", SqlDbType.NVarChar, 50)
               dbComm.Parameters.Add("@Fallout", SqlDbType.int, 4)
               dbComm.Parameters.Add("@Status", SqlDbType.NVarChar, 50)
               dbComm.Parameters.Add("@Comments", SqlDbType.NVarChar, 255)

               dbComm.Parameters("@Line").Value = cmbLine.SelectedItem.Text
               dbComm.Parameters("@Zone").Value =
cmbZone.SelectedItem.Text
               dbComm.Parameters("@Product").Value = cmbProduct.SelectedItem.Text
               dbComm.Parameters("@PFM").Value =
cmbProcessFailMode.SelectedItem.Text
               dbComm.Parameters("@Shift").Value = cmbShift.SelectedItem.Text
               dbComm.Parameters("@Originator").Value = txtOriginator.Text
               dbComm.Parameters("@CPM").Value = cmbCPM.SelectedItem.Text
               dbComm.Parameters("@LotID").Value = txtLotID.Text
               dbComm.Parameters("@NumReels").Value = txtNumReels.Text
               dbComm.Parameters("@Qty").Value = txtQty.Text
               dbComm.Parameters("@ReelID").Value = txtReelID.Text
               If txtFallout.Text = "" Then
                       dbComm.Parameters("@Fallout").Value = 0
               Else
                       dbComm.Parameters("@Fallout").Value = txtFallout.Text
               End If
               dbComm.Parameters("@Status").Value = cmbStatus.SelectedItem.Text
               dbComm.Parameters("@Comments").Value = txtComments.Text

               'Response.Write(strSQL)


       Dim iID as Integer

   Try
     objConnection.Open()
     iID = dbComm.ExecuteNonQuery()
         'Response.Write(iID)
         If (iID = 1) And (cmbStatus.SelectedItem.Value = 2) Then
               lblStatus.Text = "Record Updated. Please modify barcode."
               lblStatus3.Text = "="
           btnSubmit.Enabled = false
         Else If (iID = 1) Then
               lblStatus.Text = "Record Updated."
           btnSubmit.Enabled = false
         End If
   Catch ex As Exception
         'If ex.Message =
         Response.Write("An error has occurred: ")
     Response.Write(ex.Message)
         Response.Write("<br>RECORD NOT UPDATED. PLEASE TRY AGAIN OR REPORT
ERROR")
         Response.Write(cmbStatus.SelectedItem.Text)
     Response.End
   Finally
     If objConnection.State = ConnectionState.Open Then
       objConnection.Close()
     End If
   End Try

       If ((cmbStatus.SelectedItem.Value = 4) or
(cmbStatus.SelectedItem.Value = 5) or (cmbStatus.SelectedItem.Value =
6)) Then
                btnClose.Enabled = True
       End If


  End If

 End Sub

The drop down list is populated with 5 text values in the page load
sub. The strange thing is if I change the value in the list, I don't
get the error. It only occurs if I leave the selected item as it is
when the page loads.

I can't understand why its trying to change to type Double in the
first place. I don't have any Cast statements and the data type in the
db is nvarchar(50).

Any help is greatly appreciated!