title: TypeError: Series.name must be a hashable type
body: <pre><code>    df = pd.DataFrame(
  {
      "Index1": ["A", "A", "B", "B", "C"],
      "Index2": ["a", "b", "a", "c", "a"],
      "Param1": [1, 3, 1, 4, 2],
      "Param2": [2, 4, 3, 3, 4],
  })
  </code></pre>
  
  <p>I wanna try to <code>df.groupby(["Index1","Index2"])["Param1","Param2"].apply(lambda x:x['Param1']/x['Param2'])</code>,but got an error infomatation like that:<strong><em>TypeError: Series.name must be a hashable type</em></strong>,
  How should I fix it,thank you for your help.</p>
  
  <p>I wanna generate data like below
  <a href="https://i.stack.imgur.com/59jV2.png" rel="nofollow noreferrer">enter image description here</a></p>
  
  <pre><code>Index2  Index1  a   b   c
  0   A   0.50    0.75    nan
  1   B   0.33    nan 1.33
  2   C   0.50    nan nan
  </code></pre>
  
  <p>this format is I use <code>df.assign(dis=lambda x:x.Param1/x.Param2).groupby(['Index1','Index2'])['dis'].apply(float).unstack().reset_index()</code> generate,I wanna to know .apply(float) the way if is very dangerous </p>