Files Stored In An Array

I'm trying to open multiple files, pass them into an array and then work on them but the code below doesn't work (receive a Run-time error '9' message)

Public aFile As Variant

Sub MultipleFiles ()

MsgBox ("Please select multiple book files (holding CTRL as you click on flies) before you click open")

aFile = Application.GetOpenFilename(FileFilter:="Excel (*.xl*), *.xl*", _
Title:="Select required book level files", MultiSelect:=True)

Dim i As Integer

For i = LBound(aFile) to UBound(aFile)
Workbooks(aFile(i)).Activate
'Do something, then repeat with next file in array
Next i

End Sub