title: Why am I seeing "Error: length(rows) == 1 is not TRUE" with ddply?
body: <p>I have a data frame, say payroll, like:</p>
  
  <pre><code>payroll &lt;- read.table(text="
  AgencyName          Rate             PayBasis     Status    NumRate
  HousingAuthority    $26,843.00   Annual           Full-Time 26843.00
  HousingAuthority    $14,970.00   ProratedAnnual   Part-Time 14970.00
  HousingAuthority    $26,843.00   Annual           Full-Time 26843.00
  HousingAuthority    $14,970.00   ProratedAnnual   Part-Time 14970.00
  HousingAuthority    $13.50           Hourly           Part-Time 13.50
  HousingAuthority    $14,970.00   ProratedAnnual   Part-Time 14970.00
  HousingAuthority    $26,843.00   Annual           Full-Time 26843.00", header = TRUE)
  </code></pre>
  
  <p>The "NumRate" is actually numeric:</p>
  
  <pre><code>payroll$NumRate &lt;- as.numeric(payroll$NumRate)
  </code></pre>
  
  <p>And I'd like to get a know the max, min and mean salaries by PayBasis. I expect this to work:</p>
  
  <pre><code>ddply(payroll, "PayBasis", summarize)
  </code></pre>
  
  <p>But instead I'm getting an error: <code>Error: length(rows) == 1 is not TRUE</code></p>
  
  <p>What am I missing here?</p>