Error code : Application-defined or object-defined error ?

Now I have macro as below.

Sub test546()
    Dim a As Variant
    Dim i As Long
    a = Cells(2, 2).Resize(Cells(Rows.Count, 4).End(xlUp).Row, 4)
    For i = 1 To UBound(a): a(i, 1) = "=" & a(i, 4) & "/" & a(i, 3): Next
    Cells(2, 6).Resize(UBound(a)) = a
End Sub

But after run , it show error Application-defined or object-defined error in line Cells(2, 6).Resize(UBound(a)) = a


Please guide me about it.