title: "data type not understood" when appending DataFrames
body: <p>So I'm starting with a blank DataFrame, pulling a bunch of new dataFrames from SQL statements based on a list of table names, and then appending each one to the starting dataFrame.
  ex:</p>
  
  <pre><code>df1 = pd.DataFrame()
  for name in tableslist:
      perT = pd.read_sql_query("SELECT FROM myschema.{0} WHERE username = '{1}'.format(name,user),engine)
      df1 = df1.append(perT)
  </code></pre>
  
  <p>That returns the error: <code>data type not understood</code> in the terminal. I'm positive that it is caused by the append line (I've checked with print statements). What could cause that? It's possible for perT to be a table with no rows. Is that an issue? I've tried printing out the head of each table, and still can't determine what is causing that error/failure.</p>