PivotTable and custom Outline Code in Project 2007

[Newbie]

Hello,

I'm trying to create a Pivot Table from Project 2007 to Excel 2007 : Report
/ Visual Reports...
but when I try to add my custom Outline Code, I'm getting the error message:
"Your selection contains one or more fields that cannot be used in Visual
Reports because either the field is longer than 24 characters, contains one
of the reserved characters...
In fact : my custom field has 5 letters, not any reserved character... Same
thing for the lookup elements...
Any idea ?

Thanks for any help!