"Cannot use a scalar value as an array"

In /var/log/php.error.log:

PHP Warning: Cannot use a scalar value as an array in /var/www/drupal/sites/all/modules/domain/domain.bootstrap.inc on line 58

/**
 * Domain module bootstrap: calls all bootstrap phases.
 */
function domain_bootstrap() {
  global $_domain;
  // Initialize our global variable.
  $_domain = array();

  $phases = array(DOMAIN_BOOTSTRAP_INIT, DOMAIN_BOOTSTRAP_NAME_RESOLVE, DOMAIN_BOOTSTRAP_FULL);
  foreach ($phases as $phase) {
    // We return NULL if the domain module is not enabled.  This prevents
    // load errors during module enable / disable.
    $error = _domain_bootstrap($phase);
    if ($error === NULL) {
      $_domain['error'] = -1;
      break;
    }
    else if ($error === FALSE) {
      // watchdog() is not available here, and there may be other logging.
      // So we have to write an error message global variable and pick them up in settings,inc.
      $_domain['error'] = $phase;       <====  Line 58
      break;
    }
  }
}