Interdire valeur négative sous forme de macro

Private Sub CommandButton1_Click()
    Dim li As Byte
    Dim col As Byte
    Dim ai As Double

    li = Me.ComboBox1.ListIndex + 7
    col = Me.ComboBox2.Column(1, Me.ComboBox2.ListIndex)
    With Sheets("Données")
        Select Case col
        Case 16
            ai = .Cells(li, col - 3).Value
        Case 20 To 44
            ai = .Cells(li, col - 4).Comment.Text
        End Select
        .Cells(li, col).ClearComments
        If CDbl(Me.TextBox1.Value) >= ai Then
            .Cells(li, col).AddComment (TextBox1.Value)  
            .Cells(li, col).Value = CDbl(Me.TextBox1.Value) - ai
        Else
            MsgBox "Résultat négatif donc refusé !", vbInformation, "Attention"
        End If
    End With
    Me.TextBox1.Value = ""
End Sub