title: error reproducing stack() example in R
body: <p>When I try to reproduce the stack() example given in the raster reference manual, I get the following error:</p>
  
  <pre><code>R&gt; fn &lt;- system.file("external/test.grd", package="raster")
  R&gt; s &lt;- stack(fn, fn)
  Error in rep.int(names(x), lengths(x)) :
    unimplemented type 'NULL' in 'rep3'
  </code></pre>
  
  <p>or if I try:</p>
  
  <pre><code>R&gt; fn &lt;- system.file("external/test.grd", package="raster")
  R&gt; r &lt;- raster(fn)
  R&gt; s &lt;- stack(r, fn)
  Error in as.list.default(x) : 
    no method for coercing this S4 class to a vector
  </code></pre>
  
  <p>Has anybody an idea?!</p>