UserForm - adding exception to text box restrictions (numbers only)

I've found the below code which will prevent anything other than numbers being entered into the text boxes within my userform. However this means that a negative (-) symbol cannot be used. Is there any way I can keep the restriction but allow the negative symbol to also be used?

Private Sub OnlyNumbers()
If TypeName(Me.ActiveControl) = "TextBox" Then
    With Me.ActiveControl
        If Not IsNumeric(.Value) And .Value <> vbNullString Then
            MsgBox "Sorry, only numbers allowed"
            .Value = vbNullString
        End If
    End With
End If
End Sub