title: Pandas error when appending dataframes: invalid dtype determination in get_concat_dtype
body: <p>I have two <code>dataframes</code> with the same <code>dtypes</code>:</p>
  
  <pre><code>&gt;&gt;&gt; df1.dtypes
  Out[3]: 
  
  GUID             object
  RID               int64
  SID               int64
  Threshold       float64
  Average         float64
  dtype: object
  
  &gt;&gt;&gt; df2.dtypes
  Out[4]: 
  
  GUID           object
  RID             int64
  SID             int64
  Threshold     float64
  Average       float64
  dtype: object
  </code></pre>
  
  <p>When running
  <code>df1.append(df2)</code>
  or
  <code>pd.concat([df1,df2])</code></p>
  
  <p>I'm getting the following error:</p>
  
  <pre><code>in get_empty_dtype_and_na
      raise AssertionError("invalid dtype determination in get_concat_dtype")
  </code></pre>
  
  <p>Any idea what am I missing?</p>