title: Pandas: Setting slice of multiindexed dataframe with values of a series (aligned)
body: <p>I'm trying to set the slice of a data frame that has a multi-index with the values of a series that matches the index values. But instead the whole slice is just filled with NaN.</p>
  <p>Consider the following dataframe and series:</p>
  <pre><code>import pandas as pd
  index = pd.Index(['a', 'b', 'c'])
  multi_index = pd.MultiIndex.from_product([index, index])
  df = pd.DataFrame({'A': range(9)}, index=multi_index)
  df.sort_index(inplace=True)  # just to be sure
  ser = pd.Series([11, 22, 33], index)
  df, ser
  
       A
  a a  0
    b  1
    c  2
  b a  3
    b  4
    c  5
  c a  6
    b  7
    c  8
  
  a    11
  b    22
  c    33
  dtype: int64
  </code></pre>
  <p>Now I'm trying to set the slice with the contents of the series, but the result is unexpected:</p>
  <pre><code>df.loc[('b',), 'A'] = ser
  df
  
         A
  a a  0.0
    b  1.0
    c  2.0
  b a  NaN
    b  NaN
    c  NaN
  c a  6.0
    b  7.0
    c  8.0
  </code></pre>
  <p>And that although the indices agree:</p>
  <pre><code>df.loc[('b',), 'A'].index == ser.index
  
  array([ True,  True,  True])
  </code></pre>
  <p>This should be so simple. What am I doing wrong?</p>