Visual Basic.NET - Problema con combobox y función

Function consulta(ByVal Sql As String)

    Dim Dt As New DataTable

    Try

        Dim adapter As New NpgsqlDataAdapter(llenaSql, conexion)

        adapter.Fill(Dt)

    Catch ex As Exception

        MessageBox.Show("IMPOSIBLE COMPLETAR LA CONSULTA POR " & ex.ToString)

    End Try

    Return Dt

End Function

dt = consulta(llenaSql)

minimo_cbox.DataSource = dt

minimo_cbox.DisplayMember = "COLUMN_NAME"

minimo_cbox.SelectedIndex = 0

Private Sub minimo_cbox_SelectedIndexChanged(sender As Object, e As EventArgs) Handles minimo_cbox.SelectedIndexChanged

    conexion.Open()

    Dim valor1 As String = minimo_cbox.SelectedText

    Dim consulta1 As String = " Select min(" & valor1 & ") from datos; "

    Dim comando1 As NpgsqlCommand = New NpgsqlCommand(consulta1, conexion)

    Dim lector1 As NpgsqlDataReader = Nothing

    lector1 = comando1.ExecuteReader

    lector1.Read()

    minValor_lbl.Text = lector1(0)

    lector1.Close()

    conexion.Close()

End Sub

Private Sub minimo_cbox_SelectedIndexChanged(sender As Object, e As EventArgs) Handles minimo_cbox.SelectedIndexChanged

    conexion.Open()

    minValor_lbl.Visible = True

    Dim valor1 As String = minimo_cbox.Text

    'NOS ASEGURAMOS QUE SIN VALORES FUNCIONA

    If Not String.IsNullOrWhiteSpace(valor1) Then

        Dim consulta1 As String = " Select min(" & valor1 & ") from datos; "

        Dim comando1 As NpgsqlCommand = New NpgsqlCommand(consulta1, conexion)

        Dim lector1 As NpgsqlDataReader

        lector1 = comando1.ExecuteReader

        If lector1.Read() Then

            minValor_lbl.Text = lector1(0)

        End If

        conexion.Close()

        conexion.Open()

        Dim consulta3 As String = " Select " & valor1 & " from minimos; "

        Dim comando3 As NpgsqlCommand = New NpgsqlCommand(consulta3, conexion)

        Dim lector3 As NpgsqlDataReader

        lector3 = comando3.ExecuteReader

        lector3.Read()

        minValor_lbl.Text = minValor_lbl.Text & " SOBRE UN MÍNIMO DE " & lector3(0)

        lector1.Close()

        lector3.Close()

    End If

    conexion.Close()

End Sub