protected function backtraceStr(Event $error)
    {
        $backtrace = $error['backtrace']
            ? $error['backtrace'] // backtrace provided
            : $error->getSubject()->backtrace();
        if (\count($backtrace) < 2) {
            return '';
        }
        if ($backtrace && $error['vars']) {
            $backtrace[0]['vars'] = $error['vars'];
        }
        if ($this->cfg['emailBacktraceDumper']) {
            $str = \call_user_func($this->cfg['emailBacktraceDumper'], $backtrace);
        } else {
            $search = array(
                ")\n\n",
            );
            $replace = array(
                ")\n",
            );
            $str = \print_r($backtrace, true);
            $str = \preg_replace('#\bArray\n\(#', 'array(', $str);
            $str = \preg_replace('/\barray\s+\(\s+\)/s', 'array()', $str); // single-lineify empty arrays
            $str = \str_replace($search, $replace, $str);
            $str = \substr($str, 0, -1);
        }
        return $str;
    }