VBA USER FORM error 6 work around - vba

Question

I currently have a bunch of Combo boxes that has a option for Yes, No or N/A
Then I Have a Command button that that runs a calculation then moves to the next page
Private Sub CommandButton2_Click()
Dim a As Long, b As Long
a = IIf(Cbx1_1.Value = "Yes", 1, 0) + IIf(Cbx1_2.Value = "Yes", 1, 0) +_
IIf(Cbx1_3.Value = "Yes", 1, 0) + IIf(Cbx1_4.Value = "Yes", 1, 0)
b = 4 - IIf(Cbx1_1.Value = "N/A", 1, 0) - IIf(Cbx1_2.Value = "N/A", 1, 0)_
- IIf(Cbx1_3.Value = "N/A", 1, 0) - IIf(Cbx1_4.Value = "N/A", 1, 0)
OUTBX1.Text = Format(a / b, "00.00%")
MultiPage1.Value = 1
End Sub
Now the Only Problem that I am experiencing now is that if all the boxes are check as "N/A" then I get the 0/0 problem (Error 6 : Overflow)
I know that with Excel I would have used an ERRORIF formula to solve this. Is there a way that I can tell VBA that if:
=ERRORIF (b=0) then MultiPage1.Value = 1 else
OUTBX1.Text = Format(a / b, "00.00%")

Answer 1

Thank you to everyone for you help the final answer to this is coded as following:
Private Sub CommandButton2_Click()
Dim a As Long, b As Long
a = IIf(Cbx1_1.Value = "Yes", 1, 0) + IIf(Cbx1_2.Value = "Yes", 1, 0) + IIf(Cbx1_3.Value = "Yes", 1, 0) + IIf(Cbx1_4.Value = "Yes", 1, 0)
b = 4 - IIf(Cbx1_1.Value = "N/A", 1, 0) - IIf(Cbx1_2.Value = "N/A", 1, 0) - IIf(Cbx1_3.Value = "N/A", 1, 0) - IIf(Cbx1_4.Value = "N/A", 1, 0)
If b = 0 Then
MultiPage1.Value = 1
Else
OUTBX1.Text = Format(a / b, "00.00%")
MultiPage1.Value = 1
End If
End Sub