ValueError: All strings must be XML compatible: Unicode or ASCII, no NULL bytes

出现这个错，是因为编码的问题。

Traceback (most recent call last):
  File "/tmp/a.py", line 4, in <module>
    html5lib.parse('<p>&#1;', treebuilder='lxml')
  File "/home/simon/.virtualenvs/weasyprint/lib/python3.3/site-packages/html5lib/html5parser.py", line 28, in parse
    return p.parse(doc, encoding=encoding)
  File "/home/simon/.virtualenvs/weasyprint/lib/python3.3/site-packages/html5lib/html5parser.py", line 224, in parse
    parseMeta=parseMeta, useChardet=useChardet)
  File "/home/simon/.virtualenvs/weasyprint/lib/python3.3/site-packages/html5lib/html5parser.py", line 93, in _parse
    self.mainLoop()
  File "/home/simon/.virtualenvs/weasyprint/lib/python3.3/site-packages/html5lib/html5parser.py", line 183, in mainLoop
    new_token = phase.processCharacters(new_token)
  File "/home/simon/.virtualenvs/weasyprint/lib/python3.3/site-packages/html5lib/html5parser.py", line 991, in processCharacters
    self.tree.insertText(token["data"])
  File "/home/simon/.virtualenvs/weasyprint/lib/python3.3/site-packages/html5lib/treebuilders/_base.py", line 320, in insertText
    parent.insertText(data)
  File "/home/simon/.virtualenvs/weasyprint/lib/python3.3/site-packages/html5lib/treebuilders/etree_lxml.py", line 240, in insertText
    builder.Element.insertText(self, data, insertBefore)
  File "/home/simon/.virtualenvs/weasyprint/lib/python3.3/site-packages/html5lib/treebuilders/etree.py", line 108, in insertText
    self._element.text += data
  File "lxml.etree.pyx", line 921, in lxml.etree._Element.text.__set__ (src/lxml/lxml.etree.c:41467)
  File "apihelpers.pxi", line 652, in lxml.etree._setNodeText (src/lxml/lxml.etree.c:18888)
  File "apihelpers.pxi", line 1335, in lxml.etree._utf8 (src/lxml/lxml.etree.c:24701)
ValueError: All strings must be XML compatible: Unicode or ASCII, no NULL bytes or control characters

再生成文档过程中，突然间发现出现此错误。本来想着是通过改变编码的方式，来解决这类问题，如下所示：

p = document.add_paragraph(u"哈哈 ")
或者是：
p = document.add_paragraph(p.encode('utf-8').decode("utf-8")) 

但是我使用了上述的两种方法，错误仍然存在，后面就用了替换的方法，解决了眼前的错误（虽然目前妥协了，但是后面如果发现又更好的解决方式，会再来更新的）：

s = re.sub(u"[\\x00-\\x08\\x0b\\x0e-\\x1f\\x7f]", "", s)
p = self.doc.add_paragraph(s)