Getting Run Time Error 5 on String?

[Sully1440]

Hi All,
I'm trying to run a code to calculate Critical Path (that I found online). I can run the code for some projects, but one project results in a Run Time Error 5: Invalid Procedure Call or Argument.
I can't figure out what to change to make it work... It hangs up on a line in the Sub BackwardsPass() and the line is: ArgString = Left(ArgString, Len(ArgString) - 1)

Here is the code:

Sub RunCPM()
Call ForwardPass
Call BackwardPass
With ActiveSheet
    Set activitylist = Range("A6"): Set slacklist = Range("I6"): Set CriticalList = Range("J6")
    Set durationlist = Range("D6")
        With activitylist
            NumRows = Range(.Offset(1, 0), .Offset(1, 0).End(xlDown)).Rows.Count
        End With
    For k = 1 To NumRows
    slacklist.Offset(k, 0).Formula = "=RC[-1]-RC[-3]"
    CriticalList.Offset(k, 0).Formula = "=IF(RC[-1]=0,""YES"",)"
    Next
End With

End Sub

Sub ForwardPass()
Dim i, k, NumRows As Integer: Dim ArgString As String
With ActiveSheet
Set activitylist = Range("A6"): Set PredecessorList = Range("C6")
Set EarlyStartList = Range("E6"): Set SuccessorList = Range("K6")

With activitylist
 NumRows = Range(.Offset(1, 0), .Offset(1, 0).End(xlDown)).Rows.Count
End With
For k = 1 To NumRows
 ArgString = vbNullString
 
 If Len(PredecessorList.Offset(k, 0)) > 0 Then
 For i = 1 To k - 1
 If InStr(1, PredecessorList.Offset(k, 0), activitylist.Offset(i, 0)) > 0 Then
 ArgString = ArgString & EarlyStartList.Offset(i, 1).Address & ","
  End If
 Next
 End If
 If Len(ArgString) = 0 Then
 ArgString = 0
 End If
 EarlyStartList.Offset(k, 0).Formula = "=MAX(" & ArgString & ")"
 EarlyStartList.Offset(k, 1).Formula = "=RC[-2]+RC[-1]"
 
 Next k
End With
End Sub

Sub BackwardPass()
Dim i, k, n, NumRows As Integer: Dim ArgString As String: Dim SucString As String
Set activitylist = Range("A6"): Set PredecessorList = Range("C6")
Set EarlyStartList = Range("E6"): Set SuccessorList = Range("K6")

With ActiveSheet
With activitylist
 NumRows = Range(.Offset(1, 0), .Offset(1, 0).End(xlDown)).Rows.Count
End With
EarlyStartList.Offset(NumRows, 3).Formula = "=" & EarlyStartList.Offset(NumRows, 1).Address
EarlyStartList.Offset(NumRows, 2).Formula = "=RC[+1]-RC[-3]"
For n = 1 To NumRows - 1
 ArgString = vbNullString
 SucString = vbNullString
 k = NumRows - n
    For i = k + 1 To NumRows
        If InStr(1, PredecessorList.Offset(i, 0), activitylist.Offset(k, 0)) > 0 Then
        ArgString = ArgString & EarlyStartList.Offset(i, 2).Address & ","
        SucString = SucString & activitylist.Offset(i, 0).Address & ","","","
        End If
    Next i
'On Error Resume Next
ArgString = Left(ArgString, Len(ArgString) - 1)
EarlyStartList.Offset(k, 3).Formula = "=MIN(" & ArgString & ")"
EarlyStartList.Offset(k, 2).Formula = "=RC[+1]-RC[-3]"
SuccessorList.Offset(k, 0).Value = "=concatenate(" & SucString & ")"

Next n
End With
End Sub