title: pandas merge_asof keys must be sorted error after sorting
body: <p>I would like to merge a target column from one data frame to another. The merged data frame has many more keys and they are are close, but do not exactly match the original data frame. See an example below:</p>
  
  <p>Original data frame (FDMA)</p>
  
  <pre><code>DMA 
  130506  
  130510  
  130512  
  130555  
  130556  
  </code></pre>
  
  <p>Merged data frame (ZC)</p>
  
  <pre><code>DMA        Distance (Miles)
  1305060    303.87
  1305061    305.35
  1305062    278.80
  1305065    299.94
  1305067    291.83
  
  pd.merge_asof(FDMA,ZC[['DMA','Distance (Miles)']],on='DMA')
  </code></pre>
  
  <p>This is what I am expecting</p>
  
  <pre><code>DMA     Distance (Miles)
  130506  303.87
  130510  291.83
  130512  XXX
  130555  XXX
  130556  XXX
  </code></pre>
  
  <p>I have tried the above code and get a ValueError: right keys must be sorted error. I have sorted the values and reset the indexes, but still get the error. Any help is appreciated!</p>