title: lxml.etree.XPathEvalError: Invalid expression
body: <p>I am getting an error with Python that I am not able to understand. I have simplified my code to the very bare minimum:</p>
  
  <pre><code>response = requests.get('http://pycoders.com/archive')
  tree = html.fromstring(response.text)
  r = tree.xpath('//divass="campaign"]/a/@href')
  print(r)
  </code></pre>
  
  <p>and still get the error</p>
  
  <pre><code>Traceback (most recent call last):
  File "ultimate-1.py", line 17, in &lt;module&gt;
  r = tree.xpath('//divass="campaign"]/a/@href')
  File "lxml.etree.pyx", line 1509, in lxml.etree._Element.xpath (src/lxml/lxml.etree.c:50702)
  File "xpath.pxi", line 318, in lxml.etree.XPathElementEvaluator.__call__ (src/lxml/lxml.etree.c:145954)
  File "xpath.pxi", line 238, in lxml.etree._XPathEvaluatorBase._handle_result (src/lxml/lxml.etree.c:144962)
  File "xpath.pxi", line 224, in lxml.etree._XPathEvaluatorBase._raise_eval_error (src/lxml/lxml.etree.c:144817)
  lxml.etree.XPathEvalError: Invalid expression
  </code></pre>
  
  <p>Would anyone have an idea of where the issue is coming from? Might it be a dependencies problem? Thanks.</p>