database - DBnull から整数への変換エラー

Question

私は得るConversion from type 'DBNull' to type 'Integer' is not valid." error on the line "Dim avgObject As string = Cstr(avgCom.ExecuteScalar())

ステートメントが削除されたときにコマンドが機能し、where module_ID='" & moduleSelect & "'これを修正する方法がわかりません。誰か助けてもらえますか?

    Dim moduleSelect As String = moduleRadio.SelectedValue
    Using connection As New SqlConnection("Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\Database.mdf;Integrated Security=True;User Instance=True")
        Using avgCom As New SqlCommand("SELECT AVG(exam) FROM completed_module where module_ID='" & moduleSelect & "' ", _
                                        connection)
            connection.Open()
            Dim avgObject As Integer = CInt(avgCom.ExecuteScalar())
            Dim averageVar As String
            averageVar = avgObject.ToString
            avgLabel.Text = averageVar
        End Using