can't get drop_receiving_element to work

Any idea what's wrong with this snippet? Looks pretty simple to me.
Yet no matter what I try, the drop_receiving_element helper causes an
error.

<div id="dropFoo" class="pink box">drop here</div>
<%= drop_receiving_element('dropFoo', :hoverclass => 'hover') %>

I also tried <% drop_receiving_element :dropFoo as this seems the
preferred syntax in Raymond's AoR. Same problem, though.

Thanks,
-S.

The error message:
NoMethodError in Test#draggables2

Showing test/draggables2.html.erb where line #2 raised:

You have a nil object when you didn't expect it!
You might have expected an instance of ActiveRecord::Base.
The error occurred while evaluating nil.[]

Extracted source (around line #2):

1: <div id="dropFoo" class="pink box">drop green here</div>
2: <%= drop_receiving_element('dropFoo', :hoverclass => 'hover') %>

[...]