Visual Basic 6 - problema ciclo for e pset

Private Sub Command2_Click() 
Picture1.Cls 
Dim a, b, d, pointX, pointY As Long 
Dim CostDiv As Long 

CostDiv = 10000 
Me.DrawWidth = 5 

      For a = 0 To ListX.ListCount - 1 
      b = Val(ListX.List(a)) 
      b = b / CostDiv 
      d = Val(ListY.List(a)) 
      d = d / CostDiv   
      Picture1.PSet (b, d), vbYellow 
      Next a 
End Sub