But actually, it doesn't seem to handle this case:

```
In [9]: lxml.html.fromstring(None).text_content()
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-9-38e4f657a77d> in <module>()
----> 1 lxml.html.fromstring(None).text_content()

/Users/jbau/.virtualenv/mitx/lib/python2.7/site-packages/lxml/html/__init__.pyc in fromstring(html, base_url, parser, **kw)
    658     if parser is None:
    659         parser = html_parser
--> 660     start = html[:10].lstrip().lower()
    661     if start.startswith('<html') or start.startswith('<!doctype'):
    662         # Looks like a full HTML document

TypeError: 'NoneType' object has no attribute '__getitem__'
```