Runtime Error '9' Subscript Out Of Range: VBA Code to Append data from Multiple Workbooks into a single Workbook

Question

I have about 10 workbooks in a testing folder. All of them have a sheet labeled Detail. I need to copy out data from that sheet in the range B21:BT (stop after hitting a blank row-- as there the number of rows can change for each workbook.) I would like to paste all of this data into a workbook that is already created labeled Master.

Here is what I have so far but im hitting a runtime '9' subscript out of range error:

Sub CopyRange()
Application.ScreenUpdating = False
Dim wkbDest As Workbook
Dim wkbSource As Workbook
Set wkbDest = ThisWorkbook
Const strPath As String = "G:\GLOBALPRGM\STAFF\Testing\"
ChDir strPath
strExtension = Dir("*.xlsx*")
Do While strExtension <> ""
    Set wkbSource = Workbooks.Open(strPath & strExtension)
    With wkbSource
        .Sheets("Detail").Range("B20:BT" & .Sheets("Detail").Range("B" & Rows.Count).End(xlUp).Row).Copy wkbDest.Sheets("Master").Cells(Rows.Count, "A").End(xlUp).Offset(1, 0)
        .Close savechanges:=False
    End With
    strExtension = Dir
Loop
Application.ScreenUpdating = True
End Sub

any help cracking this one is much appreciated.