title: R studio: Error in mutate()`: !Problem while computing `timestamp = ymd_hms(timestamp)
body: <p>Getting below error when i'm trying to run the code</p>
  <pre><code>tweets &lt;- bind_rows(tweets_julia %&gt;% mutate(person = &quot;Julia&quot;), tweets_dave %&gt;% mutate(person = &quot;David&quot;)) %&gt;% mutate(timestamp = ymd_hms(timestamp))
  </code></pre>
  <p>Error in <code>mutate()</code>:
  ! Problem while computing <code>timestamp = ymd_hms(timestamp)</code>.
  Caused by error in <code>as.character()</code>:
  ! cannot coerce type 'closure' to vector of type 'character'</p>