Can't create root node if multiple roots and primary key have multiple columns

When trying to create new root node, the following exception is thrown.

PHP Fatal error: Uncaught exception 'Doctrine_Tree_Exception' with message 'Node must have a root id set or must be persistent and have a single-valued numeric primary key in order to be created as a root node. Automatic assignment of a root id on transient/new records is no longer supported.'

This only happens when hasManyRoots is true, and table have multiple columns in primary key.

Example:

$category = new Menu();
$category->name = 'Menu Root';
$category->language_id = 1;
$category->root_id = 23;
$category->save();

$treeObject = Doctrine::getTable('Menu')->getTree();
$treeObject->createRoot($category);

and menu table is defined as follows:

$this->hasColumn('id', 'integer', 8, array(
'type' => 'integer',
'primary' => true,
'autoincrement' => true,
'length' => '8',
));
$this->hasColumn('language_id', 'integer', 8, array(
'type' => 'integer',
'primary' => true,
'length' => '8',
));

If i remove primary option from language_id column, everything works as it should.