errore su codice

[pietro09]

mi sapete dire perchè questo codice va in errore?

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
	Dim a$ = ""

	Response.Write(prova(a))

End Sub

Function prova(ByVal a As String) As String
	Return Replace(a, ",", ".").ToLower
End Function