Входная строка и мела неверный формат

Private Sub txtq_TextChanged(sender As Object, e As EventArgs) Handles txtt.TextChanged, txtq.TextChanged, txtI.TextChanged
    If txtI.Enabled = False Then

        Dim I As Double
        Dim q As Double = Convert.ToDouble(txtq.Text)
        Dim t As Double = Convert.ToDouble(txtt.Text)
        I = q / t
        txtI.Text = Convert.ToString(I)
    End If

    If txtq.Enabled = False Then
        Dim I As Double = Convert.ToDouble(txtI.Text)
        Dim q As Double
        Dim t As Double = Convert.ToDouble(txtt.Text)
        q = I * t
        txtq.Text = Convert.ToString(q)
    End If
End Sub

При обработке пер q выдает входная строка имела неверный формат