Unable to get the RowRange property of the PivotTable class

I have a VB.NET program that open an Existing worksheet containing some pivot tables. For each pivot table I want get the RowRange property:

dim oPt as Excel.PivotTable
dim oRng as excel.Range

for each oPt in oWs.PivotTables
oRng = oPt .RowRange
.
.
.
next

Sometimes, The istruction "oPt .RowRange" raise this exception:

Run-time exception thrown : System.Runtime.InteropServices.COMException - Unable to get the RowRange property of the PivotTable class

WHY!?!?