title: AttributeError: 'Series' object has no attribute 'to_numeric'
body: <p>I'm trying to sort dataframe by values. got an AttributeError: 'Series' object has no attribute 'to_numeric'. version '0.20.3', so to numeric should work, but not. Please help. </p>
  
  <pre><code>import pandas as pd
      tables = pd.read_html("https://www.sec.gov/Archives/edgar/data/949012/000156761919015285/xslForm13F_X01/form13fInfoTable.xml")
      len(tables)
      ren=tables[3]
      ren.drop(ren.index[[0,1,2]], inplace=True)
      ren.to_numeric(ren[3], errors='coerce')
      #ren[3].convert_objects(convert_numeric=True)
      ren.sort_values(by=[3],ascending=False)
  </code></pre>