missing profile2 uid values

Apologies in advance for not being able to get my head around this. I am trying to get a d6->d7 migration started, and in the old site have profile fields which I would like to migrate into Profile2 fields on the new site. Here's what I have so far:

// User Migration class
class ukcUserProfileMigration extends Migration {
  // class constructor
  public function __construct() {
    parent::__construct(MigrateGroup::getInstance('allukcusers'));
    $this->description = t('Migrate Drupal 6 user profiles');
    
    $this->map = new MigrateSQLMap($this->machineName,
      array(
        'uid' => array(
          'type' => 'int',
          'unsigned' => TRUE,
          'not null' => TRUE,
        )
      ),
      MigrateDestinationProfile2::getKeySchema()
    );
    
    $query = Database::getConnection('default', 'legacy')
          ->select('users', 'u')
          ->fields('u', array('uid'))
          ->condition('u.uid', 1, '>');
   
    $this->source = new MigrateSourceSQL($query);
    
    $this->destination = new MigrateDestinationProfile2('main');
 
    
    // Create the field mappings
    $this->addFieldMapping('field_account_first_name', 'profile_first_name');
    

  }

  public function prepareRow($current_row) {
    $profile_fields = Database::getConnection('default', 'legacy')
        ->select('profile_values', 'pv')
        ->fields('pv', array('fid', 'value'))
        ->condition('pv.uid', $current_row->uid)
        ->condition('pv.fid', 3, '>')
        ->orderBy('pv.fid', 'ASC')
        ->execute()
        ->fetchAllKeyed();
    // print_r ($profile_fields[4]);
    foreach ($profile_fields as $row) {
        $current_row->profile_first_name[] = $profile_fields[4];
    }
  }
}  

I'll add additional field mappings once I have the thing working.

This is putting everyone's first name into the field_data_field_account_first_name table as expected, but it is not putting the associated uid into the profile table.

Appreciate any suggestions.