title: Pandas - KeyError on merge
body: <p>I'm trying to merge 2 dataframes. I'm using the Jupyter notebook and pandas dataframes. My two dfs look like this:</p>
  
  <pre><code>gbdf.dtypes:
  product_name            object
  Quantity               float64
  Product_id               int64
  product_group1           int64
  product_group1_name     object
  product_group2           int64
  product_group2_name     object
  packing_unit            object
  packing_amount           int64
  dtype: object
  
  
  trns.dtypes:
  Store_id           int64
  Date              object
  Price              int64
  Net price          int64
  Purchase price     int64
  Hour               int64
  product_id         int64
  Quantity           int64
  dtype: object
  </code></pre>
  
  <p>Yet, when I try to run </p>
  
  <pre><code>gbdfprice = gbdf.merge(gbdf, trns, left_on = 'Product_id', right_on = 'product_id')
  </code></pre>
  
  <p>I get</p>
  
  <pre><code>KeyError: 'product_id'
  </code></pre>
  
  <p>Any idea why?</p>