better exception message

modules/elasticsearch/src/main/java/org/elasticsearch/search/dfs/CachedDfSource.java

@@ -48,7 +48,7 @@ public class CachedDfSource extends Searcher {
     public int docFreq(Term term) {
         int df = dfs.dfMap().get(term);
         if (df == -1) {
-            throw new IllegalArgumentException("df for term " + term.text() + " not available");
+            throw new IllegalArgumentException("df for term " + term + " not available");
         }
         return df;
     }