title: strata() from "sampling" returns an error: arguments imply differing number of rows
body: <p>I have a data frame that looks like this:</p>
  
  <pre><code>'data.frame':   1090 obs. of  8 variables:
   $ id            : chr  "INC000000209241" "INC000000218488" "INC000000218982" "INC000000225646" ...
   $ service.type  : chr  "Incident" "Incident" "Incident" "Incident" ...
   $ priority      : chr  "Critical" "Critical" "Critical" "Critical" ...
  </code></pre>
  
  <p>I order the data as follows:</p>
  
  <pre><code>data &lt;- data[order(data$priority),]
  </code></pre>
  
  <p>I have been changing priority to factors etc but regardless what I try, when I try running the below:</p>
  
  <pre><code>s = strata(data,c("priority"),size=c(0,0,1,5))
  </code></pre>
  
  <p>I always get the following error:</p>
  
  <pre><code>Error in data.frame(..., check.names = FALSE) : 
    arguments imply differing number of rows: 0, 1
  </code></pre>
  
  <p>I tried debugging the function to see whether I could tell why this error is raised (but I couldn't make sense of the code). The error was raised at this stage of executing the strata() function:</p>
  
  <pre><code>debug: r = cbind(r, i)
  </code></pre>
  
  <p>Thank you very much for all your help!</p>