Run Time Error 1004 During Sort Method

I encounter a runtime error '1004' if the "Invoices" sheet is not selected when I run this procedure. The last line of the code is one which is highlighted when I debug.

Sub ProcessData()
Dim aiOldRows() As Integer, aiNewRows() As Integer ' Arrays of new/old rows
Dim rngRaw As Range 'Data entry area
Dim rngInvoices As Range 'Invoices range
Dim rngOpenPoint As Range 'Top-left corner of data entry area
Set rngOpenPoint = ThisWorkbook.Worksheets("Data Entry").Range("a3")
Set rngRaw = Range(rngOpenPoint, rngOpenPoint.End(xlDown).End(xlToRight))
FindNew aiOldRows, aiNewRows, rngRaw
InvoiceSequence aiOldRows, rngRaw
Set rngInvoices = Range(ThisWorkbook.Worksheets("Invoices").Range("A2"), _
ThisWorkbook.Worksheets("Invoices").Range("A3").End(xlDown).End(xlToRight))
rngInvoices.Sort Key1:=Range("M2"), Order1:=xlAscending
End Sub

You'll notice that there are two other procedures (FindNew & InvoiceSequence) being called by this procedure. I don't think those have anything to do with the error, but I can provide the code for those if needed. Oh, and one other secondary question. To declare the ranges rngRaw & rngInvoices I pick the top-left cell of the data and then do:.....................