求助大神，一直报错1004，有时可运行，有时报错。

abccr2003

Private Sub CommandButton5_Click()

Dim b()
ReDim b(1 To 100)
al = Sheet4.Application.CountA(Range("a:a"))
For n = 2 To al
    ta = 0
    For m = 2 To 28 Step 2
    Set frng1 = Sheet3.Range("a2:g100").Find(Sheet4.Cells(n, m), , , xlWhole)
     If Not frng1 Is Nothing Then
        b(m) = frng1.Offset(0, 6).Value * Sheet4.Cells(n, m + 1)
        X = b(m)
        ta = ta + X
    End If
    Next
    Sheet4.Cells(n, "av") = ta
Next
rs = Sheet4.Application.CountA([a:a])
Sheet4.Range("a1:av" & rs).Copy
Sheet6.Activate
Sheet6.UsedRange.ClearContents
Sheet6.Paste cells(a1)
End Sub

有时在运行Sheet4.Range("a1:av" & rs).Copy时报，有时在Sheet6.Paste时报。我是VBA初学者，业务上需要。请大神指点