title: ValueError: Inferred frequency None from passed values does not conform to passed frequency MS
body: <p>I am trying to create month frequency index for time series analysis
  The <code>date</code>variable is a timestamp starts from <code>2018-10-18 10:50:10</code> till <code>2019-11-25 15:09:33</code>. I am getting the error in subject line
  i tried removing the nulls but that did not help.
  New to python programming, any help</p>
  
  <pre><code>df=pd.read_excel("Source_Data.xlsx",index_col="activation date",parse_dates=True)
  
  df.rename(
      columns={
          "activation date":"Date",
          "Estimate Volumes in HL (ABI)":"Volume"
      },
      inplace=True
  )
  
  dataset=df.filter(["Date","Volume"],axis=1)
  
  dataset.index.freq="MS"
  </code></pre>