diff --git a/libraries/stdlib/src/kotlin/collections/MutableCollections.kt b/libraries/stdlib/src/kotlin/collections/MutableCollections.kt
index 1a99bebe612..ba221cfd04c 100644
--- a/libraries/stdlib/src/kotlin/collections/MutableCollections.kt
+++ b/libraries/stdlib/src/kotlin/collections/MutableCollections.kt
@@ -4,7 +4,10 @@ package kotlin
  * Adds all elements of the given *iterable* to this [[MutableCollection]]
  */
 public fun <T> MutableCollection<in T>.addAll(iterable: Iterable<T>): Unit {
-    for (e in iterable) add(e)
+    when (iterable) {
+        is Collection -> addAll(iterable)
+        else -> for (e in iterable) add(e)
+    }
 }
 
 public fun <T> MutableCollection<in T>.addAll(stream: Stream<T>): Unit {