subscript out of range error

I am trying to refresh a pivot table on a separate workbook using vba.
Here is the code:

Dim pt as PivotTable
Set pt = Workbooks("ControlLists.xls").Worksheets("Detail").PivotTables("PivotTable1")
pt.RefreshTable

I get the error about being out of range on the 'Set' line of code.

Why?