form_for on a nested singleton

Hello-

I have a resource for Orders with a nested singleton for Address. I
find that is I use the form_for syntax like this:

form_for([@order, @address)

I get an error that says:

undefined method `order_addresses_path' for #<ActionView::Base:
0xb6afcefc>

if I change the form_for syntax to look like this:

form_for(@address, :url => order_address_path(@order))

then it starts to work. My question is, is this the expected behavior
of form_for or am I doing something wrong?

Regards,
Eric