[Zope] Using aquisition inside a dtml method of a product ?

Inside a product my index_html is set to "index_html=HTMLFile('index_html',globals())" The index_html.dtml calls <dtml-var standard_html_header>. This DTML method is available in the top-level hierarchy. However When I call index_html Zope complains with a KeyError/standard_html_header. When I remove the "<dtml-var standard_html_header>" call everything works fine. Any idea why aquisition won't work in this case ? Andreas