For Next stops at 16

The following code fails when I set the upper range of the For Next loop to any number greater than 16. The error: "Run-time error '9': Subscript out of range". The statement that it throws the error on is:

If Vol(i, 1) <> OldVol(i, 1) Then

It works fine for any upper range up to and including 16; but 17 or greater throws the error.

Any help appreciated.

Code Module:

Dim NewVol As Variant
Dim OldVol As Variant
Dim Vol As Variant
Dim RT As Variant
Dim BSRT As Variant
Dim ASRT As Variant
Dim BSize As Variant
Dim ASize As Variant
Dim Start As Variant

Sub Update()
'OldVol = Range("ab55:ab57").Value
'NewVol = Range("ab55:ab57").Value
BSize = Range("n55:n70").Value
ASize = Range("q55:q70").Value
BSRT = Range("ad55:ad70").Value
ASRT = Range("ae55:ae70").Value
Vol = Range("z55:z70").Value

If Start = 0 Then
Exit Sub
End If
'if TimeValue("00:00:08")

Range("ad54").Select

For i = 1 To 16
If Vol(i, 1) <> OldVol(i, 1) Then
BSRT(i, 1) = BSize(i, 1) + BSRT(i, 1)
ActiveCell.offset(i, 0).Value = BSRT(i, 1)

ASRT(i, 1) = ASize(i, 1) + ASRT(i, 1)
ActiveCell.offset(i, 1).Value = ASRT(i, 1)

OldVol(i, 1) = Vol(i, 1)
End If
Next

ActiveCell.offset(-4, 0).Value = ActiveCell.offset(-4, 0).Value + 1
Application.OnTime Now + TimeValue("00:00:01"), "Update"


End Sub