title: ValueError: labels not contained in axis
body: <p>I have data of the form :</p>
  
  <p><code>IMP_START_TIME                         IMP_CLR_TIME        SERV_OR_IOR_ID
  0       2017-02-28 23:59:32.8730  2017-03-01 00:11:28.7550    -1447310116
  1       2017-03-01 00:00:09.1820  2017-03-01 00:01:06.9120     1673545041
  ...                          ...                       ...            ...
  266863  2017-03-01 04:05:28.2200                       nan     2108335332
  266866  2017-03-01 13:10:01.1600                       nan     -724153592</code></p>
  
  <p>I want to remove all the rows which have "nan" in the IMP_CLR_TIME column. For this I wrote the following code :</p>
  
  <pre><code>df = pd.read_csv(r'C:\Users\SIA_1_3_2017.csv',low_memory=False)
  SID_ST_CT_col = df[['IMP_START_TIME','IMP_CLR_TIME','SERV_OR_IOR_ID']]
  
  SID_ST_CT_str = SID_ST_CT_col.astype(str)                
  
  
  SID_ST_CT_str.drop(SID_ST_CT_str.loc[SID_ST_CT_str['IMP_CLR_TIME']=='nan'])
  </code></pre>
  
  <p>But I am getting the following error :</p>
  
  <pre><code>ValueError: labels ['IMP_START_TIME' 'IMP_CLR_TIME' 'SERV_OR_IOR_ID'] not contained in axis
  </code></pre>
  
  <p>When I print the rows which have 'nan' in the IMP_CLR_TIME column using the following command, it works.But I am unable to figure out why I am getting such an error when I try to delete the same rows.</p>