title: merge() missing 1 required positional argument: 'right'
body: <p>I am doing a tutorial and I have a problem:
  <strong>My code:</strong></p>
  
  <pre><code>import html5lib 
  import quandl
  import pandas as pd
  import pickle
  
  pd.read_html("https://simple.wikipedia.org/wiki/List_of_U.S._states")
  main_df = pd.DataFrame()
  fiddy_states = pd.read_html('https://simple.wikipedia.org/wiki/List_of_U.S._states')
  for abbv in fiddy_states[0][0][1:]:
      query = "FMAC/HPI_"+str(abbv)
      df = quandl.get(query)
  
  if main_df.empty:
      main_df = df 
  else:
      main_df = pd.merge (main_df , df, how = "right")
  print(pd.merge(main_df))
  </code></pre>
  
  <p><strong>and my error is</strong>: </p>
  
  <pre><code>TypeError: merge() missing 1 required positional argument: 'right'
  </code></pre>
  
  <p>what's wrong? </p>