title: HDFStore error appending - "Cannot serialize the column"
body: <p>I have a dataframe, df:</p>
  
  <pre><code>    datetime                      bid      ask     bidvolume  askvolume
  0   2007-03-30 21:00:00.332000   1.9682   1.9678       4         0.8
  </code></pre>
  
  <p>Trying to append this to a new datastore. The datastore does not exist so I use the following to create and append the data;</p>
  
  <pre><code>store = pd.HDFStore(storePath,mode='w')
  store.append('data',df)
  store.close()
  </code></pre>
  
  <p>I get this error: on the <code>store.append</code> line.</p>
  
  <pre><code>TypeError: Cannot serialize the column [bid] because
  its data contents are [floating] object dtype
  </code></pre>
  
  <p>How do I get the data to store properly?</p>