diff --git a/src/library/scala/collection/Iterator.scala b/src/library/scala/collection/Iterator.scala
index 572dc4078f63..2966d271d5de 100644
--- a/src/library/scala/collection/Iterator.scala
+++ b/src/library/scala/collection/Iterator.scala
@@ -991,7 +991,8 @@ object Iterator extends IterableFactory[Iterator] {
     */
   def newBuilder[A]: Builder[A, Iterator[A]] =
     new ImmutableBuilder[A, Iterator[A]](empty[A]) {
-      override def addOne(elem: A): this.type = { elems = elems ++ single(elem); this }
+      override def addOne(elem: A): this.type             = { elems ++= single(elem); this }
+      override def addAll(xs: IterableOnce[A]): this.type = { elems ++= xs.iterator;  this }
     }
 
   /** Creates iterator that produces the results of some element computation a number of times.