When I try and copy a selection of cells my code throws the following code

Runtime Error 424: Object Required

Any Ideas? Here is part of my code

For a = 1 To ACount
mainworkbook.Activate
Sheets("List").Cells(a*20, 1).Select
Range(ActiveCell, ActiveCell.Offset(-19, 5)).Select
Selection.Copy ' THIS IS WHERE THE ERROR OCCURS
secondaryworkbook.Activate
Sheets(a).Cells(1, 1).Select
Selection.Paste
Next