ASP.Net Error: operation is not valid due to the current state of the object

My code is to increment the Compno number automatically. Previously this code works perfectly, but now this code is not working. when i fecth the query n datatable view its not incrementing, but when i copy the queruy its working perfectly. More over i dont why this message is coming in the oracle command property "operation is not valid due to the current state of the object. oracle". Can you give me a perfect solution to my problem. With this i also attached the coding

This Query is the problem 

SELECT Cast(NVL(Max(comp_no),0)+ 1  AS  Int) FROM elec_cmp_no_upd

    Protected Sub btnsubmit_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnsubmit.Click
        Try
            Dim view As common = New common()
            Dim dt As DataTable
            dt = rep.getComplaintNo()
            Dim compno As String
            Dim updno As String
            If txtdept.Text = "" Or txtdept.Text = "SELECT DEPARTMENT" Then
                Rfvdept.ErrorMessage = "*"
            ElseIf txtunit.Text = "" Or txtunit.Text = "SELECT UNIT" Then
                Rfvunit.ErrorMessage = "*"
            Else
                updno = dt.Rows(0)(0).ToString()
                compno = "COMP" + dt.Rows(0)(0).ToString()
                Dim unit As String = ddlunit.SelectedItem.Text.ToString()
                Dim dept As String = txtdept.Text
                Dim comp As String = ddlcomp.SelectedItem.Text.ToString()
                Dim remarks As String = txtremarks.Text
                Dim priority As String = ddlprior.SelectedItem.Text.ToString()
                Dim rname As String = txtrname.Text
                Dim dte As String = oradate.ToOracleDate(System.DateTime.Now.ToString())
                rep.RegComplaint(compno, unit, dept, comp, remarks, priority, rname, dte)
                rep.UpdateComplaintNo(updno)
                Dim dtlog As DataTable = view.checkuser(CStr((Session("User"))))
                view.CreateLogFile(dtlog.Rows(0)(0).ToString(), dtlog.Rows(0)(6).ToString(), "Complaint Registration Form", "Complaint Registered Successfully", dte)
                SuccessMessage.Show()
            End If
        Catch ex As Exception
            MsgBox(ex)
        End Try
    End Sub

    Public Function getComplaintNo() As DataTable
        getconnection()
        Using con As New OracleConnection(co)
            Using cmd As New OracleCommand("SELECT Cast(NVL(Max(comp_no),0)+ 1  AS  Int) FROM elec_cmp_no_upd")
                Using sda As New OracleDataAdapter()
                    cmd.Connection = con
                    sda.SelectCommand = cmd
                    cmd.CommandType = CommandType.Text
                    Using dt As New DataTable()
                        sda.Fill(dt)
                        Return dt
                    End Using
                End Using
            End Using
        End Using
    End Function