[Zope] <dtml-tree> from within a product

[chrisf]

I have the following code in a product

dtml_method index_html

<dtml-var standard_html_header>
<dtml-let url="REQUEST.PARENTS[1]">

   <dtml-in "display(param=url)">
   <dtml-var "getId()">
   </dtml-in>

   <dtml-tree  branches="display(param=url)">
   <dtml-var "getId()">
   </dtml-tree>
</dtml-let>
<dtml-var standard_html_footer>

python script

return (param.objectValues('Image'))

output is fine for the upper <dtml-in> loop but NO TREE

WHY