title: dplyr : Error in mutate_impl(.data, dots) : argument 'times' incorrect
body: <p>I have a tibble as :</p>
  
  <pre><code># A tibble: 2 × 2
    read_seq unique_id
       &lt;chr&gt;     &lt;dbl&gt;
  1 AATTGGCC         1
  2   GGGTTT         2
  </code></pre>
  
  <p>I want to create a new variable containing a string of the same size as read_seq. I did  that but have an error:</p>
  
  <pre><code>&gt; r %&gt;% mutate(y=paste(rep("H",width(read_seq)),sep=""))
  Error in mutate_impl(.data, dots) : argument 'times' incorrect
  </code></pre>
  
  <p>When I only try to catch read_seq width it works :</p>
  
  <pre><code>&gt; r %&gt;% mutate(y=width(read_seq))
  # A tibble: 2 × 3
    read_seq unique_id     y
       &lt;chr&gt;     &lt;dbl&gt; &lt;int&gt;
  1 AATTGGCC         1     8
  2   GGGTTT         2     6
  </code></pre>
  
  <p>Here's the dput() of the example for reproducibility :</p>
  
  <pre><code>r &lt;- structure(list(read_seq = c("AATTGGCC", "GGGTTT"), unique_id = c(1, 
  2)), class = c("tbl_df", "tbl", "data.frame"), row.names = c(NA, 
  -2L), .Names = c("read_seq", "unique_id"))
  </code></pre>