What did I do? (Select Method of Range Class Failed )

I've been running some code successfully for a few weeks now, the only thing
that I've changed is that I now copy a range of data from the Main Sheet to
a Working Sheet before I start manipulating the data. I haven't change the
actual code below that now gives me the ERROR "Selection Method of Range
class Failed"


Set Rng = Worksheets(SheetName2).UsedRange.Rows

'I Get the Error trying to execute the next line
Application.Worksheets(SheetName2).Range("A" & First_Row, "G" &
Rng.Rows.Count).Select