title: invalid factor level, NA generated
body: <p>Beginner here.</p>
  
  <p>I'm trying to append a row to an existing data frame. I made a new vector (below) and then tried to bind it to the original data frame. Received some warning messages.   </p>
  
  <pre><code>#append new row
  new_row &lt;- c(50, 22, "Roberto", "Bolano", "MALE", "2003-07-15")
  
  #bind to original data frame
  writers_df_large &lt;- rbind(writers_df, new_row)
  </code></pre>
  
  <p>Warning messages:
  1: In <code>[&lt;-.factor</code>(<code>*tmp*</code>, ri, value = "Roberto") :
  invalid factor level, NA generated</p>
  
  <p>2: In <code>[&lt;-.factor</code>(<code>*tmp*</code>, ri, value = "Bolano") :
  invalid factor level, NA generated</p>
  
  <p>3: In <code>[&lt;-.factor</code>(<code>*tmp*</code>, ri, value = "2003-07-15") :
  invalid factor level, NA generated</p>
  
  <p><code>StringsAsFactors = FALSE</code>  should solve this, but I'm not sure how to go about it.  </p>