title: df ['X'].unique() and TypeError: unhashable type: 'numpy.ndarray'
body: <p>all,</p>
  
  <p>I have a column in  a dataframe that looks like this:</p>
  
  <pre><code>allHoldingsFund['BrokerMixed']
  Out[419]: 
  78         ML
  81       CITI
  92         ML
  173      CITI
  235        ML
  262        ML
  264        ML
  25617      GS
  25621    CITI
  25644    CITI
  25723      GS
  25778    CITI
  25786    CITI
  25793      GS
  25797    CITI
  Name: BrokerMixed, Length: 2554, dtype: object
  </code></pre>
  
  <p>Although the column is an object. I am not able to group by that column or even extract the unique values of that column. For example when I do: </p>
  
  <pre><code>allHoldingsFund['BrokerMixed'].unique()
  </code></pre>
  
  <p>I get an error</p>
  
  <pre><code>     uniques = table.unique(values)
    File "pandas/_libs/hashtable_class_helper.pxi", line 1340, in pandas._libs.hashtable.PyObjectHashTable.unique
  TypeError: unhashable type: 'numpy.ndarray'
  </code></pre>
  
  <p>I also get an error when I do group by.</p>
  
  <p>Any help is welcome. Thank you</p>