title: Error in Dataset$text : object of type 'closure' is not subsettable
body: <p>I'm trying to perform a sentiment analysis of Tweeter..But at one stage I`ve got </p>
  
  <blockquote>
    <p>"Error in Dataset$text : object of type 'closure' is not subsettable"
    :</p>
  </blockquote>
  
  <pre><code>Dataset &lt;- stack    
  Dataset$text &lt;- as.factor(Dataset$text) 
  scores &lt;- score.sentiment(Dataset$text, pos.words, neg.words, .progress='text')
  write.csv(scores, file=paste(searchterm, '_scores.csv'), row.names=TRUE)
  </code></pre>