title: TypeError: ('sequence item 0: expected string, numpy.int64 found', u'occurred at index 1')
body: <p>I am calculating the frequency of each sequence in <code>df</code>:</p>
  
  <pre><code>VD_1    VD_2    VD_2
  35000   35090   31550
  35000   35090   31550
  35099   45097   
  35099   45097   
  35099   45097
  </code></pre>
  
  <p>If I do run the code given below, I get an error <code>TypeError: ('sequence item 0: expected string, numpy.int64 found', u'occurred at index 1')</code>. In fact, the code works fine on the other dataset, but here it fails:</p>
  
  <pre><code>df['data'] = df.apply(lambda x: '/'.join(x.dropna()), axis=1)
  df = df.data.value_counts().rename_axis('count').reset_index()
  df
  </code></pre>
  
  <p>The result should be this one:</p>
  
  <pre><code>data                count
  35000/35090/31550   2
  35099/45097         1
  </code></pre>