使用 Markdown 处理文本时报错

$content = 'xxxxxx';
$markdown->convertMarkdownToHtml($content);

运行时报错了，看错误提示应该是参数类型造成的，有没有遇到类似错误的大大？
错误信息如下：

{"error":
    {"type":"ErrorException",
     "message":"Value for Core.Encoding is of invalid type, should be istring invoked on line 76 in file ROOT_DIR\\vendor\\mews\\purifier\\src\\Mews\\Purifier\\Purifier.php",
     "file":"ROOT_DIR\\vendor\\ezyang\\htmlpurifier\\library\\HTMLPurifier\\Config.php","line":892
    }
}