Excel macro does not work when used as an addin

I have written a VBA macro that copies/merges multiple workbooks into one and it works as desired. But when I try to create this macro as an addin, and run the code by using my shortcut key of Ctrl+Shift+M, the code fails with no error. The code is as below:

Dim wbk As Workbook
Dim wbk1 As Workbook
Set wbk1 = Workbooks.Add
Set wbk1 = ThisWorkbook
Dim Filename As String
Dim Path As String
Dim Rowvar As Integer
Rowvar = 1
Titlelabcnt = 1
Path = "C:\"
Filename = Dir(Path & "*.xlsx")

Do While Len(Filename) > 0
Set wbk = Workbooks.Open(Path & Filename)
Dim Lastcol As Long
Dim LastRow As Long
Lastcol = ActiveSheet.Cells.Find(What:="*", After:="A2", SearchOrder:=xlByColumns, searchdirection:=xlPrevious).Column
LastRow = ActiveSheet.Cells.Find(What:="*", After:="A2", SearchOrder:=xlByRows, searchdirection:=xlPrevious).Row
Range("A2").Resize(LastRow, Lastcol).Select
Selection.Copy Destination:=wbk1.Sheets("sheet1").Range("A" & Rowvar)
Rowvar = Rowvar + LastRow
wbk.Close True
Filename = Dir
End Sub

The above code stops when used as an addin right at the following line:

Set wbk = Workbooks.open(Path & Filename)

Can you provide any suggestions on what to change such that it works like an addin as well?