group_by missing items

I use the acts_as_tree plugin, and I try to display a list of the
elements w group_by

in my controller
...
    companies = Company.find :all, conditions => Company.visible_by
(User.current), :include => :parent
    respond_to do |format|
      format.html {
        @company_tree = companies.group_by {|p| p.parent || p}
      }

I have the company ids : [ 1, 2, 3, 4, 5, 6]
with the following tree structure : 1 children ->[ 2 , 3] 2 ->
children [4, 5, 6] and 3 has no child

the companies.group by gives me correctly an Hash with
[key company 1 -> value [company 2, company 3]
key company 2 -> value [company 4, company 5, company 6] ]

but no key company 3 ... is it because the group is empty (no
child) ... how can I get the key company 2, with an empty array as
value ? is it possible ?

thanks for your help