xpath取src属性返回空是什么原因？

爬取百度文库的时候获取不到标签内的属性值

问题相关代码，请勿粘贴截图

url = "https://www.sohu.com/a/399253135_99943404%22
url_status_code = requests.get(url).status_code
url_status_text = requests.get(url).text

html = etree.HTML(url_status_text)

p = html.xpath('//*[@id="mp-editor"]')
for a in p:
src = a.xpath('.//img/@src')
print(src)

运行结果及报错内容

标签取到img时正常返回
for a in p:
src = a.xpath('.//img')

增加属性值时返回空列表
for a in p:
src = a.xpath('.//img/@src')

我想要达到的结果

怎样可以渠道img标签内的src属性值