VB.net Excel Add-in Find values on sheet and don't go past the end of the sheet

Hi
I am using the following VB.net code in my Excel Add-in to find all instances of a word on a spreadsheet.
The loop doesn't end at the end of the sheet  but rather just carries in. What extra code can I include
to not go past the end of the sheet?

                    
Dim blnFindError As Boolean = False

                    oFindWhat = "Total"
                    strSourceRange = oSourceWorkBook.Sheets(oSourceSheet).Cells.Address

                    Do Until blnFindError = True

                        Try
                            oFindAddress = oSourceWorkBook.Sheets(oSourceSheet).Range(strSourceRange).Find(
        What:=oFindWhat, After:=oSourceWorkBook.Sheets(oSourceSheet).Range(oFindAfterAddress),
        LookIn:=Excel.XlFindLookIn.xlValues, LookAt:=Excel.XlLookAt.xlPart, SearchOrder:=oSearchOrder,
        SearchDirection:=Excel.XlSearchDirection.xlNext).Address
                        Catch ex As Exception
                            blnFindError = True
                        End Try

                        oFindAfterAddress = oFindAddress 


                    Loop

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