title: Error in packets in spplot using lattice
body: <p>I am plotting 2 maps side by side in lattice using an spplot command, using the simple default choropleth style color fill in sp. The syntax I am using is identical to this example:</p>
  
  <pre><code>library(sp)
  library(rgdal)
  library(lattice)
  
  spplot(nc, c("SID74", "SID79"), names.attr = c("1974","1979"),
      colorkey=list(space="bottom"), scales = list(draw = TRUE),
      main = "SIDS (sudden infant death syndrome) in North Carolina",
      sp.layout = list(arrow), as.table = TRUE)
  </code></pre>
  
  <p>The maps plot perfectly and exactly as intended side by side. However, I get a warning written on top of each of the maps as follows:</p>
  
  <p>'Error using packet 1 length must be a 'unit' object'(on chart 1)</p>
  
  <p>'Error using packet 2 length must be a 'unit' object' (on chart 2)</p>
  
  <p>I apologize that I cannot give the data here to help troubleshoot. However, I was hoping someone might be able to point me in the right direction of figuring out what is going wrong. </p>
  
  <p>Note: both maps chart perfectly fine as individual maps using the equivalent of:</p>
  
  <pre><code>spplot(nc,"SID74")
  </code></pre>
  
  <p>Clearly the problem is with the lattice view, but I am not familiar enough (despite having tried to look in Sarkar  ch8) with packets to be able to know what is going wrong.</p>
  
  <p>As an alternative, given that my charts are mapping as required, is there a way simply to turn off these warnings?</p>
  
  <p>Many thanks for any help</p>