title: "TypeError: 'DataFrame' objects are mutable, thus they cannot be hashed" while sorting pandas dataframe index
body: <p>I have a following dataframe <code>h</code>:</p>
  
  <pre><code>In [24]: h.head()
  Out[24]: 
                   alpha1  alpha2    gamma1  gamma2       chi2min gender  age
  filename                                                                   
  F35_HC_532d.dat  0.0000   0.000       NaN    0.00  1.000000e+25      F   35
  M48_HC_551d.dat  0.7353   3.943  0.425922    0.15  2.072617e+01      M   48
  M24_HC_458d.dat  0.7777   4.754  0.463753    0.15  1.390893e+01      M   24
  M48_HC_552d.dat  0.7633   3.672  0.394370    0.15  1.965052e+01      M   48
  M40_HC_506d.dat  0.7793   3.271  0.513597    0.20  1.089716e+01      M   40
  </code></pre>
  
  <p>I am trying to sort the dataframe index according to age values:</p>
  
  <pre><code>In [25]: h.sort_index(h.sort_values('age'))
  </code></pre>
  
  <p>This throws an error:</p>
  
  <pre><code>TypeError: 'DataFrame' objects are mutable, thus they cannot be hashed
  </code></pre>
  
  <p>What am I missing? Any ideas?</p>