Why does nested loop cause a Run Time Error 13? - excel

Question

I've written a macro that structures a lot of data and saving me for manual punching of numbers etc. The macro is written in stages were each part have been tested isolated and then integrated in loops or to the main code it self - this way each piece of code is tested so it functions accordingly to my intentions.
The goal for this part of code is to cycle through each row in the sheet, if the value of column 9, row I is different from the column 9, row I-1, then it will insert a new row (sum row). This action will be preformed in each worksheet, hence nested loop. When I wrote this macro isolated, without nesting loops, it functioned well.
Edit: To clearify, the code insert a row if and only if Cells(I, PrGr) = Cells(I - 1, PrGr) are unequal. Therefore I ask if they are the same, if they are so, do nothing - else, insert a row (i.e. Cells(I, PrGr) = Cells(I - 1, PrGr) are not equal.)
Running it in a nested loop causes a Run Time Error 13, type mismatch on the line with "If Cells(I, PrGr) = Cells(I - 1, PrGr) Then". In debugger, when I force it to continue, it does what it is supposed to do - creating the sum rows for in every sheet. This happens regardless of which sheet I set as the starting sheet.
I've tried to change the logic of the loop by testing with For Each ws In This.. and with the logic I have now For J = 1 to WS_ant. Both causes the error. I also searched around for clues, but non has come up with any solution appropriate for this problem.
Does someone have a clue to what is happening here, and how to fix it?
The code:
Sub OI_SJ()
'Selects the first sheet
Sheets(1).Activate
'Loop through sheets
Dim J As Integer
Dim WS_ant As Integer
Dim starting_ws As Worksheet
Set starting_ws = ActiveSheet 'remember which worksheet is active in the beginning
WS_ant = ThisWorkbook.Worksheets.Count
For J = 1 To WS_ant
ThisWorkbook.Worksheets(J).Activate
'If sheet = GRL, then terminate
If ThisWorkbook.Worksheets(J).Name = "LL - Sv" Or ThisWorkbook.Worksheets(J).Name = "RM - Sv" Then
'do something later
ElseIf ThisWorkbook.Worksheets(J).Name = "GRL" Then GoTo Term5
Else
Dim I As Integer
Dim PrGr As Long
PrGr = 9
Set aktivtark = ThisWorkbook.Worksheets(J)
With aktivtark
sistekolonne = aktivtark.Cells(1, .Columns.Count).End(xlToLeft).Column
sisterad = aktivtark.Cells(.Rows.Count, "A").End(xlUp).Row
End With
Application.CutCopyMode = False
Application.ScreenUpdating = False
With aktivtark.Range("A6", Cells(sisterad, sistekolonne))
.RowHeight = 15
End With
Rows(1).RowHeight = 24
For I = 6 To sisterad + 153
If Cells(I, PrGr) = Cells(I - 1, PrGr) Then
ElseIf Cells(I, PrGr) = "PrGr" Then
Else
Rows(I).EntireRow.Insert
Cells(I, 12) = "SUM " & Cells(I - 1, PrGr).Value
Cells(I, 33).Formula = "=SUMIF(R7C9:R1500C9,MID(RC12,5,255),R7C:R1500C)"
Cells(I, 34).Formula = "=SUMIF(R7C9:R1500C9,MID(RC12,5,255),R7C:R1500C)"
Cells(I, 35).Formula = "=SUMIF(R7C9:R1500C9,MID(RC12,5,255),R7C:R1500C)"
Cells(I, 36).Formula = "=SUMIF(R7C9:R1500C9,MID(RC12,5,255),R7C:R1500C)"
Cells(I, 37).Formula = "=SUMIF(R7C9:R1500C9,MID(RC12,5,255),R7C:R1500C)"
Cells(I, 38).Formula = "=SUMIF(R7C9:R1500C9,MID(RC12,5,255),R7C:R1500C)"
Cells(I, 39).Formula = "=SUMIF(R7C9:R1500C9,MID(RC12,5,255),R7C:R1500C)"
Cells(I, 40).Formula = "=SUMIF(R7C9:R1500C9,MID(RC12,5,255),R7C:R1500C)"
Cells(I, 41).Formula = "=SUMIF(R7C9:R1500C9,MID(RC12,5,255),R7C:R1500C)"
Cells(I, 42).Formula = "=SUMIF(R7C9:R1500C9,MID(RC12,5,255),R7C:R1500C)"
Cells(I, 43).Formula = "=SUMIF(R7C9:R1500C9,MID(RC12,5,255),R7C:R1500C)"
Cells(I, 44).Formula = "=SUMIF(R7C9:R1500C9,MID(RC12,5,255),R7C:R1500C)"
Rows(I).RowHeight = 17.25
I = I + 1
End If
Next
Application.CutCopyMode = True
Application.ScreenUpdating = True
End If
Cycle:
Next
Term5:
starting_ws.Activate 'activate the worksheet that was originally active
End Sub

Answer 1

With the insight from Vitaliy Prushak's post, I've changed that bit of code from If Cells(I, PrGr) = Cells(I - 1, PrGr) Then to If Cells(I, PrGr).Text = Cells(I - 1, PrGr).Text Then, since the cells contain text. This pushed the problem further down in the code, but it was solved with the same solution.
The macro now runs as it should, thanks for the help.