title: undefined columns selected：restart Rstudio and run the same code I used before I encounter this
body: <p>here is my data,</p>
  
  <pre><code>&gt; head(annotation)
    seqname    feature start   end
  1       1 transcript 11869 14409
  2       1       exon 11869 12227
  3       1       exon 12613 12721
  4       1       exon 13221 14409
  5       1 transcript 11872 14412
  6       1       exon 11872 12227
  </code></pre>
  
  <p>I want to extract the data with feature <code>transcript</code>, so I used</p>
  
  <pre><code>&gt; annotation &lt;- annotation[annotation$feature=='transcript']
  Error in [.data.frame(annotation, annotation$feature == "transcript") : 
    undefined columns selected
  </code></pre>
  
  <p>I just used this command yesterday afternoon, and it functioned normally. 
  I don't know why it gives me an error now.</p>
  
  <pre><code>&gt; annotation$feature=='transcript'
     [1]  TRUE FALSE FALSE FALSE  TRUE FALSE FALSE FALSE  TRUE FALSE FALSE FALSE
    [13] FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE  TRUE FALSE FALSE
    [25] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE  TRUE FALSE
    [37] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE  TRUE FALSE FALSE FALSE
    [49] FALSE FALSE FALSE FALSE FALSE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE
    [61] FALSE FALSE FALSE FALSE FALSE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE
  </code></pre>
  
  <p>Please help, thanks.</p>