title: Error: `by` required, because the data sources have no common variables
body: <p>I am trying to apply the codes to my data in this link 
  <a href="https://www.tidytextmining.com/sentiment.html#sentiment-analysis-with-inner-join" rel="nofollow noreferrer">https://www.tidytextmining.com/sentiment.html#sentiment-analysis-with-inner-join</a></p>
  
  <p>The code in the book is </p>
  
  <pre><code>nrc_joy &lt;- get_sentiments("nrc") %&gt;% 
    filter(sentiment == "joy")
  tidy_books %&gt;%
    filter(book == "Emma") %&gt;%
    inner_join(nrc_joy) %&gt;%
    count(word, sort = TRUE)
  </code></pre>
  
  <p>I wrote it like the following (excluded "filter" because I have just filenames and words columns in my data)</p>
  
  <pre><code>nrc_joy &lt;- get_sentiments("nrc") %&gt;% 
    filter(sentiment == "joy")
  </code></pre>
  
  <p><code>abc %&gt;%
        inner_join(nrc_joy ) %&gt;% 
        count(word, sort = TRUE)
  </code>
  I get this error:</p>
  
  <blockquote>
    <p>Error: <code>by</code> required, because the data sources have no common variables</p>
  </blockquote>
  
  <p>Any ideas how to deal with it?</p>