"Could not save to map table" for every item

Hi there --
I'm using migrate to move a bunch of data from a homegrown JSP/Postgres cms to D7, and all is going fairly well--I'm connecting to the Postgres database fine, and am able to import items without trouble. However, for some reason my migrate_map_* tables aren't getting populated. When I start the migration with
drush mi MigrationName
every single item that comes in triggers the following error messages:

Undefined property: stdClass::$style                                                                                                                                                    [error]
File /var/www/html/d7/sites/all/modules/contrib/migrate/includes/source.inc, line 257(file: /var/www/html/d7/sites/all/modules/contrib/migrate/includes/source.inc, line 257)
Undefined property: stdClass::$style
File /var/www/html/d7/sites/all/modules/contrib/migrate/plugins/sources/sqlmap.inc, line 301(file: /var/www/html/d7/sites/all/modules/contrib/migrate/plugins/sources/sqlmap.inc, line 301)Could not save to map table due to NULL value for key field style

This has been true for both nodes and terms (the old system had a taxonomy system), so I'm pretty sure it's not something I've done wrong in a particular Migration class, since I've set up two. I'm thinking it has something to do with using Postgres as my source? Anyway, here's one of my Migration classes for reference:

class ClearinghouseMigration extends Migration {
  public function __construct() {
    parent::__construct(MigrateGroup::getInstance('clearinghouse'));
    
    $this->description = t('Migrate records and documents');
    
    // Make sure the 2 term migrations happen first.
    $this->dependencies = array('ChCat', 'ChTag');
    
    $this->map = new MigrateSQLMap($this->machineName,
      array(
        'style' => array('type' => 'int',
                          'length' => 10,
                          'unsigned' => TRUE,
                          'not null' => TRUE,
                          'description' => 'Source record ID',
                        )
      ),
      MigrateDestinationNode::getKeySchema()
    );

    $query = Database::getConnection('default', 'phi_migrate')
      ->select('article', 'a')
      ->fields('a', array('art_author_fname', 'art_pubmonth', 'art_pubyear', 'art_date', 'art_volume', 'art_publisher', 'art_pub', 'art_http', 'art_binary', 'art_fulltext', 'art_order_details', 'art_entrydate', 'art_hardcopy', 'art_softcopy'));
    $query->join('resource', 'r', 'a.res_id=r.res_id');
    $query->fields('r', array('res_name', 'res_abstract'));

    // Source
    $this->source = new MigrateSourceSQL($query, array(), NULL, array('map_joinable' => FALSE));
    
    // Destination
    $this->destination = new MigrateDestinationNode('articles_commentaries');
    
    // Mappings
    $this->addFieldMapping('title', 'res_name');
    $this->addFieldMapping('body', 'res_abstract');
    $this->addFieldMapping('field_author', 'art_author_fname');
    $this->addFieldMapping('field_volume', 'art_volume');
    $this->addFieldMapping('field_publisher', 'art_publisher');
    $this->addFieldMapping('field_pub', 'art_pub');
    $this->addFieldMapping('field_url', 'art_http');
    $this->addFieldMapping('field_full_text_of_article', 'art_fulltext');
    $this->addFieldMapping('field_ordering_info', 'art_order_details');
    
  }
}

Thanks for any light you can shed. And thanks for this awesome module!
Ben