title: MergeError: right can only have one index
body: <p>I have a large dataframe df (applicationstartdate as a index) and a series app_3M, I want to merge together.</p>
  <p>The series looks like:</p>
  <pre><code>New_ID  applicationstartdate
  1001    2021-02-28              0.0
  1003    2021-01-31              0.0
          2021-02-28              2.0
          2021-03-31              2.0
          2021-04-30              1.0
  Name: N_App_3M, dtype: float64
  </code></pre>
  <p>I want to merge the last (numerical) column to df on New_ID and applicationstartdate.</p>
  <p>I tried:</p>
  <pre><code>df = pd.merge_asof(df, app_3M, direction=&quot;nearest&quot;, left_index=True, right_index=True)
  </code></pre>
  <p>which resulted in:</p>
  <blockquote>
  <p>MergeError: right can only have one index</p>
  </blockquote>
  <p>Help would be appreciated.</p>