C# / VB.NET - form child e TransparencyKey

buongiorno,
con il seguente codice apro una form trasparente

Private Sub Form2_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Me.BackColor = Color.DarkGray 'crea form2 trasparente
TransparencyKey = Color.DarkGray
End Sub

se però la rendo figlia di una form mdi perdo la trasparenza

Private Sub Form2_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Me.MdiParent = FMdi
Me.BackColor = Color.DarkGray 'crea form2 trasparente
TransparencyKey = Color.DarkGray
End Sub

come potrei risolvere ? Grazie