Schema fix - panels pages

Schema module warning
panels_node.did is part of the primary key but is not specified to be 'not null'.

Primary key cant be null and why it is not serial?

panels_node.install

  $schema['panels_node'] = array(
    'fields' => array(
      'nid' => array(
        'type' => 'int',
        'not null' => TRUE,
        'default' => 0,
      ),
      'css_id' => array(
        'type' => 'varchar',
        'length' => '255',
      ),
      'did' => array(
        'type' => 'int',
+        'not null' => TRUE,
      ),
    ),
    'primary key' => array('did'),
  );