diff --git a/lib/natural/tfidf/tfidf.js b/lib/natural/tfidf/tfidf.js
index a8ecba4fe..dd1ba0a06 100755
--- a/lib/natural/tfidf/tfidf.js
+++ b/lib/natural/tfidf/tfidf.js
@@ -98,7 +98,7 @@ TfIdf.prototype.tfidf = function(terms, d) {
 TfIdf.prototype.listTerms = function(d) {
     var terms = [];
 
-    for(term in this.documents[d]) {
+    for(var term in this.documents[d]) {
 	terms.push({term: term, tfidf: this.tfidf(term, d)})
     }