title: Python Pandas pd.merge_asof: TypeError: 'NoneType' object is not callable
body: <p>I just discovered pandas <code>merge_asof</code> function and trying to get it to work.<br>
  Below is some sample code that reproduces the error I am seeing:</p>
  
  <pre><code>d1 = pd.DataFrame.from_dict({'id':[646327.0, 645714.0, 645462.0],
                               'units1':[1.7723982570833334, 1.7723982570833334, 5.772398257083333]})
  
  d2 = pd.DataFrame.from_dict({'id':[645462.0, 645714.0, 646327.0, 645462.0, 645714.0,
  645462.0,
  645714.0,
  645462.0,
  645714.0,
  645462.0], 'units2':[0.7723988920833333, 0.7723988920833333, 0.7723988920833333, 3.7723988920833333,
  3.7723988920833333,
  4.772398892083333,
  4.772398892083333,
  5.772398892083333,
  5.772398892083333,
  6.772398892083333]})
  </code></pre>
  
  <p>And when I try to use <code>pd.merge_asof</code>:</p>
  
  <pre><code>d3 = pd.merge_asof(d1, d2, by = 'id', left_on = 'units1', right_on = 'units2',
                  direction='nearest')
  </code></pre>
  
  <p>I get an error a <code>Nonetype error</code>.<br>
  Anyone know what's going on here? Is my syntax incorrect?</p>