diff --git a/lib/index.js b/lib/index.js
index 1e4dbac0..83bc5471 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -151,11 +151,15 @@ lunr.Index.prototype.add = function (doc, emitEvent) {
     var fieldTokens = this.pipeline.run(lunr.tokenizer(doc[field.name]))
 
     docTokens[field.name] = fieldTokens
-    lunr.SortedSet.prototype.add.apply(allDocumentTokens, fieldTokens)
+
+    for (var i = 0; i < fieldTokens.length; i++) {
+      var token = fieldTokens[i]
+      allDocumentTokens.add(token)
+      this.corpusTokens.add(token)
+    }
   }, this)
 
   this.documentStore.set(docRef, allDocumentTokens)
-  lunr.SortedSet.prototype.add.apply(this.corpusTokens, allDocumentTokens.toArray())
 
   for (var i = 0; i < allDocumentTokens.length; i++) {
     var token = allDocumentTokens.elements[i]
diff --git a/lib/sorted_set.js b/lib/sorted_set.js
index 2f0cd6d2..a8422940 100644
--- a/lib/sorted_set.js
+++ b/lib/sorted_set.js
@@ -224,7 +224,9 @@ lunr.SortedSet.prototype.union = function (otherSet) {
 
   unionSet = longSet.clone()
 
-  unionSet.add.apply(unionSet, shortSet.toArray())
+  for(var i = 0, shortSetElements = shortSet.toArray(); i < shortSetElements.length; i++){
+    unionSet.add(shortSetElements[i])
+  }
 
   return unionSet
 }