Collection of Partials, :as Option No Good, TemplateError

I'm trying to render a collection of partials using :as. It raises an
undefined local variable exception. Not sure why:

~/delivery-log/app/views/packages $ ls -1
_package.html.erb
_package_list.html.erb
index.html.erb

index.html.erb:
<%=
render :partial=>'package_list', :collection=>@packages, :as=>:package
%>

_package_list.html.erb:
<tr>
  <td><%= package.upc %></td>
</tr>

Seems simple, but it only raises the aforementioned exception:

ActionView::TemplateError (undefined local variable or method
`package' for #<ActionView::Base:0xb7589ac8>) on line #2 of packages/
_package_list.html.erb:
1: <tr>
2: <td><%= package.upc %></td>
3: </tr>

Why not use :locals => ?

Sure there a workarounds, but they don't explain why :as won't work.

You're right, of course (except for locals being a workaround), and it
seems like they should work the same or similar, but since :as is
nearly impossible to Google for I recommend whatever workable method
presents itself. That is, all other things being equal, whichever
method is better documented is the stronger one. But that's just me.