title: filtering pandas dataframe and avoid nan
body: <p>Ok I have this pandas dataframe</p>
  
  <pre><code>import pandas
  dfp=pandas.DataFrame([5,10,1,7,13,4,5,7,8,10,11,3])
  </code></pre>
  
  <p>And i want to create a second data frame with the rows that have a value greater than 5, thereby:</p>
  
  <pre><code>dfp2=dfp[dfp&gt;5]
  </code></pre>
  
  <p>My problem is that I obtain this result:</p>
  
  <pre><code>    0
  0   NaN
  1   10
  2   NaN
  3   7
  4   13
  5   NaN
  6   NaN
  7   7
  8   8
  9   10
  10  11
  11  NaN
  </code></pre>
  
  <p>And what I want is this other result:</p>
  
  <pre><code>    0
  0   10
  1   7
  2   13
  3   7
  4   8
  5   10
  6   11
  </code></pre>
  
  <p>What is wrong with my code?</p>
  
  <p>Thanks a lot</p>