Taxonomy advanced and required option

I have a field defined this way:

array(
  'type'       => 'taxonomy_advanced',
  'field_type' => 'select_tree',
  'required'   => true,
),

And I have a tree like this:

• Item level 1
  • Item level 2
  • Item level 2
    • Item level 3
    • Item level 3

The problem is that with required equal true, the user can only choose one item of level 3. What I need is that the user can choose any item in the hierarchy. Without setting the field as required, the user can select any option, but I need to force the user to choose one.