title: Create NaN column in pandas DataFrame
body: <p>I saw the following example to illustrate how to create a NaN column in a  DataFrame.</p>
  
  <pre><code>import pandas as pd
  import numpy as np
  import math
  import copy
  import datetime as dt
  
  """
  Accepts a list of symbols along with start and end date
  Returns the Event Matrix which is a pandas Datamatrix
  Event matrix has the following structure :
      |IBM |GOOG|XOM |MSFT| GS | JP |
  (d1)|nan |nan | 1  |nan |nan | 1  |
  (d2)|nan | 1  |nan |nan |nan |nan |
  (d3)| 1  |nan | 1  |nan | 1  |nan |
  (d4)|nan |  1 |nan | 1  |nan |nan |
  ...................................
  ...................................
  Also, d1 = start date
  nan = no information about any event.
  1 = status bit(positively confirms the event occurence)
  """
  
  def find_events(ls_symbols, d_data):
      ''' Finding the event dataframe '''
      df_close = d_data['actual_close']
      ts_market = df_close['SPY']
  
      print "Finding Events"
  
      # Creating an empty dataframe
      df_events = copy.deepcopy(df_close) # type &lt;class 'pandas.core.frame.DataFrame'&gt;
      df_events = df_events * np.NAN # &lt;&lt; why it works here
  </code></pre>
  
  <p>I try to duplicate the method as follows:</p>
  
  <pre><code>import numpy as np
  import pandas as pd
  from pandas import Series, DataFrame
  
  data = {'state': ['Ohio', 'Ohio', 'Ohio', 'Nevada', 'Nevada'],
          'year': [2000, 2001, 2002, 2001, 2002],
          'pop': [1.5, 1.7, 3.6, 2.4, 2.9]}
  frame = DataFrame(data)
  frame = frame * np.NAN # TypeError: can't multiply sequence by non-int of type 'float'
  </code></pre>
  
  <p>Q> Why it doesn't work here now?</p>