Warning of check_plain()

Warning: htmlspecialchars() expects parameter 1 to be string, array given in check_plain()
caused in _email_verify_check_internal by

      $metadata = stream_get_meta_data($connect);
      if ($debugging_mode) {
        $debugging_text[] = t(
          'A connection was made to "%smtp", but a positive response was not recieved. There may be additional information from the server: "%metadata". Verification is stopping here (!date_time).',
          array(
            '%smtp' => $smtp,
            '%metadata' => $metadata,
            '!date_time' => format_date(time(), $date_time_format),
          )
        );
      }

because stream_get_meta_data returns an array. So I changed
'%metadata' => $metadata,
to
'%metadata' => serialize($metadata),