[Zope] import of "aq_base" from "Acquisition" is unauthorized. You are not allowed to access 'aq_base' in this context

I've add "from Acquisition import aq_base" in python script in "portal_skins/custom/" When I test a page, I've this error : import of "aq_base" from "Acquisition" is unauthorized. You are not allowed to access 'aq_base' in this context Why this ? How to resolve it ? I search information about it in Zope Api and Google, I found nothing :( Thanks for your help, Stéphane