VBA Variable Workbook Reference Runtime-Error 9 - vba

Question

I'm trying to do a VlookUp with two seperate workbooks. The main workbook opens the supporting workbook right at the beginning like this:
Public Sub GetFile()
Dim MySourceSheet As Variant, wb As Workbook
'Opens window to choose file from
MySourceSheet = Application.GetOpenFilename(FileFilter:= _
"Excel Files (*.XLS; *.XLSX), *.XLS; *.XLSX", Title:="Select file to
retrieve data from.")
If MySourceSheet = False Then Exit Sub
Set wb = Workbooks.Open(MySourceSheet)
End Sub
Throughout the module I reference the document a few times, and then there is this part were the error occurs.
Private Sub VlookupToBeReviewed()
Dim LastColumn As Long
Workbooks("09_PR Status Custodians.xlsm").Activate 'change name
'inserts new column
With Sheets("Prio_Custodians")
LastColumn = Cells(2, .Columns.Count).End(xlToLeft).Column 'change row! 'defines last column
For i = LastColumn To LastColumn Step -2 'moves two columns to the left
.Columns(i).Insert
.Columns(i).ColumnWidth = 10
Next
End With
'puts in the date
Cells(3, LastColumn).Value = Date
'sets up the variables for the vlookup
Dim rw As Long, col As Range
Dim twb As Workbook
'sets up the workbooks
Set twb = ThisWorkbook
Set col = Workbooks(MySourceSheet).Sheets("Documents to be reviewed").Range("A:B")
'vlookup function
With twb.Sheets("Sheet1")
For rw = 3 To .Cells(Rows.Count, 1).End(xlUp).Row
.Cells(rw, LastColumn) = Application.VLookup(.Cells(rw, 2).Value2, col, 2, False)
Next rw
End With
End Sub
Error:
Set col = Workbooks(MySourceSheet).Sheets("Documents to be reviewed").Range("A:B")
Spits out the
Run-time error '9' (Subscript out of range)
However, when I move my courser over MySourceSheet it shows the correct workbook as defined earlier in the module. If I put the name of the workbook manually with the .xls extension, it works all perfectly fine.I tried looking for an answer all over the web but nothing could quite fix it. I'd very much appreciate some help! :)