VBA Find method errors on Excel Mac 2011

The following method is working just fine in Excel for Windows but on the Mac i get a 448 runtime error "cannot find argument"

Set aCell = Worksheets(SheetName).Rows(1).Find(What:=HeaderName, LookIn:=xlValues, _
      LookAt:=xlWhole, SearchOrder:=xlByRows, SearchDirection:=xlNext, _
      MatchCase:=False, SearchFormat:=False).Activate

Any ideas?