problem adding 2 values ​​in 2 columns

[RuneDefour]

everything runs smoothly except at step. adding 2 the value in column 5 and the value in column 7. the result is not correct and the whole program crashes and exits. what's my mistake? the bold is where it goes wrong

Private Sub Worksheet_Change(ByVal Target As Range)
Dim lastRow As Long
Dim i As Long
lastRow = ActiveSheet.Cells(Rows.Count, 1).End(xlUp).Row 
For i = 11 To lastRow 
   If Cells(i, 8) = 0 Then 
      If Cells(i, 7) < 1 And Cells(i, 5) < Cells(i, 6) Then 
         Cells(i, 5).Interior.Color = RGB(255, 153, 153) 
         ElseIf Cells(i, 5) >= Cells(i, 6) Then 
         Cells(i, 5).Interior.Color = RGB(255, 255, 255) 
         ElseIf Cells(i, 7) >= 1 Then 
         Cells(i, 5).Interior.Color = RGB(255, 204, 153) 
      End If
   ElseIf Cells(i, 8) >= 1 Then 
   Cells(i, 5).Value = Cells(i, 5).Value + Cells(i, 7).Value 
   Cells(i, 7).Value = 0 
   Cells(i, 5).Interior.ColorIndex = xlColorIndexNone 
   End If
    Next i
End Sub