Type mismatch with Offset function Excel VBA

Question

I'm new to VBA macros, the function has to read a formula, write the result to resultCell and go down to have other values in the formula. Everything was fine, but it only counted the first values (BJ4-AV4), so that it counts further BJ5, BJ6, BJ7... I used offset(i,0) so it should go down. I already used offset to write the result, but when I entered it to go from cell to cell in the formula, it broke. I hope the code helps make sense of the nonsense I wrote

Sub avg()
    Dim firstCell, resultCell As Range
    Dim i As Integer

    Set firstCell = Worksheets(1).Range("BJ4")
    Set resultCell = Worksheets(1).Range("CA4")

    For i = 0 To 502
        ' go to 12 left
        If firstCell.Offset(i, -12).Value > 0 Then
            resultCell.Offset(i, 0).Value = (firstCell.Offset(i, 0).Value - firstCell.Offset(i, -12).Value) / 6
        ElseIf firstCell.Offset(i, -10).Value > 0 Then
        ' if previous is null go to 10 left
            resultCell.Offset(i, 0).Value = (firstCell.Offset(i, 0).Value - firstCell.Offset(i, -10).Value) / 5
        ElseIf firstCell.Offset(i, -14).Value > 0 Then
        ' if previous is null go to 14 left
            resultCell.Offset(i, 0).Value = (firstCell.Offset(i, 0).Value - firstCell.Offset(i, -14).Value) / 7
        End If
    Next i
End Sub

table example