title: Sampling a SpatialPolygonsDataFrame with spsample() results in error from seq.default()
body: <p>I am trying to use <code>spsample()</code> from <code>sp</code> to sample points that are within the borders of Belgium, using a <code>SpatialPolygonsDataFrame</code> from <a href="http://www.gadm.org/" rel="nofollow">GDAM</a>. This results in an error from a <code>seq.default()</code>-call.</p>
  
  <pre><code>be &lt;- readRDS(gzcon(url('http://biogeo.ucdavis.edu/data/gadm2.8/rds/BEL_adm0.rds')))
  spsample(be, type="hexagonal", cellsize=10)
  </code></pre>
  
  <blockquote>
    <p>Error in <code>seq.default(ll[1], ur[1] - dx/2, dx)</code> : 
          wrong sign in 'by' argument</p>
  </blockquote>
  
  <pre><code>str(be)
  
  # Formal class 'SpatialPolygonsDataFrame' [package "sp"] with 5 slots
  #   ..@ data       :'data.frame':   1 obs. of  68 variables:
  #   .. ..$ OBJECTID     : int 1
  # [snip]
  #   .. ..$ LDC          : chr ""
  #   ..@ polygons   :List of 1
  #   .. ..$ :Formal class 'Polygons' [package "sp"] with 5 slots
  #   .. .. .. ..@ Polygons :List of 3
  #   .. .. .. .. ..$ :Formal class 'Polygon' [package "sp"] with 5 slots
  #   .. .. .. .. .. .. ..@ labpt  : num [1:2] 3.37 51.36
  # [snip]
  </code></pre>
  
  <p>Further attempts:</p>
  
  <ul>
  <li>Using the Shapefile-version from GDAM, results in the same error.</li>
  <li>Converting to a <code>SpatialPolygons</code>-object first, results in the same error.</li>
  </ul>
  
  <hr>
  
  <pre><code>packageVersion('sp')
  
  # [1] ‘1.2.3’
  </code></pre>