title: Dplyr - Error: column '' has unsupported type
body: <p>I have a odd issue when using dplyr on a data.frame to compute the number of missing observations for each group of a character variable. This creates the error "Error: column "" has unsupported type.</p>
  
  <p>To replicate it I have created a subset. The subset rdata file is available here: 
  <a href="http://www.gisintersect.com/dftest.rdata" rel="nofollow">rdata file including dftest data.frame</a>
  First. Using the subset I have provided, the code:</p>
  
  <pre><code>dftest %&gt;%
    group_by(file) %&gt;%
    summarise(missings=sum(is.na(v131)))
  </code></pre>
  
  <p>Will create the error:
  Error: column 'file' has unsupported type</p>
  
  <p>The str(dftest) returns:</p>
  
  <pre><code>'data.frame':   756345 obs. of  2 variables:
   $ file: atomic  bjir31fl.dta bjir31fl.dta bjir31fl.dta bjir31fl.dta ...
    ..- attr(*, "levels")= chr 
   $ v131: Factor w/ 330 levels "not of benin",..: 6 6 6 6 1 1 1 9 9 9 ...
  </code></pre>
  
  <p>However, taking a subset of the subset, and running the dplyr command again, will create the expected output.</p>
  
  <pre><code>dftest &lt;- dftest[1:756345,]
  dftest %&gt;%
    group_by(file) %&gt;%
    summarise(missings=sum(is.na(v131)))
  </code></pre>
  
  <p>The str(dftest) now returns:</p>
  
  <pre><code>'data.frame':   756345 obs. of  2 variables:
   $ file: chr  "bjir31fl.dta" "bjir31fl.dta" "bjir31fl.dta" "bjir31fl.dta" ...
   $ v131: Factor w/ 330 levels "not of benin",..: 6 6 6 6 1 1 1 9 9 9 ...
  </code></pre>
  
  <p>Anyone have any suggestions about what might cause this error, and what to do about it. In my original file I have 300 variables, and dplyr states that most of these are of unsupported type.</p>
  
  <p>Thanks.</p>