title: What does the rpart "Error in as.character(x) : cannot coerce type 'builtin' to vector of type 'character' " message mean?
body: <p>I've been banging my head against <code>rpart</code> for a few days now (trying to make classification trees for this dataset that I have), and I think it's time to ask a lifeline at this point :-)  I'm sure it's something silly that I'm not seeing, but here's what I've been doing:</p>
  
  <pre><code>EuropeWater &lt;- read.csv(file=paste("/Users/artessaniccola/Documents/",
                         "Magic Briefcase/CityTypology/Europe_water.csv",sep=""))
  library(rpart)
  attach(EuropeWater)
  names(EuropeWater)
  [1] "City"          "waterpercapita_m3" "water_class"       "population"       
  [5] "GDPpercapita"  "area_km2"          "populationdensity" "climate"            
  EuropeWater$water_class &lt;- factor(EuropeWater$water_class, levels=1:3, 
                                    labels=c("Low", "Medium", "High"))
  EuropeWater$climate &lt;- factor(EuropeWater$climate, levels=2:4, 
                                labels=c("Arid", "Warm temperate", "Snow"))
  EuropeWater_tree &lt;- rpart(EuropeWater$water_class ~ 
                 population+GDPpercapita + area_km2 + populationdensity + 
                 EuropeWater$climate, 
                 data=EuropeWater, method=class)   
  Error in as.character(x) : 
            cannot coerce type 'builtin' to vector of type 'character'
  </code></pre>
  
  <p>and for the life of me, I can't figure out what the Error is about.</p>