diff --git a/core/src/main/scala/org/apache/spark/util/hash/OpenHashMap.scala b/core/src/main/scala/org/apache/spark/util/hash/OpenHashMap.scala
index 4eb52158b02d428bce2e2a824fa5648c9fc5e8ed..e53551ced62ec7ada495a94486a3066643346e53 100644
--- a/core/src/main/scala/org/apache/spark/util/hash/OpenHashMap.scala
+++ b/core/src/main/scala/org/apache/spark/util/hash/OpenHashMap.scala
@@ -27,7 +27,7 @@ package org.apache.spark.util.hash
  */
 private[spark]
 class OpenHashMap[K >: Null : ClassManifest, @specialized(Long, Int, Double) V: ClassManifest](
-  var keySet: OpenHashSet[K], var _values: Array[V])
+  val keySet: OpenHashSet[K], var _values: Array[V])
   extends Iterable[(K, V)]
   with Serializable {
 
diff --git a/core/src/main/scala/org/apache/spark/util/hash/PrimitiveKeyOpenHashMap.scala b/core/src/main/scala/org/apache/spark/util/hash/PrimitiveKeyOpenHashMap.scala
index 1bf2554fb77301ea887f4582cd0d4d7544b6bc0e..08fc74e5da6ba72b0baa36ccfa80e91e1b0fa35e 100644
--- a/core/src/main/scala/org/apache/spark/util/hash/PrimitiveKeyOpenHashMap.scala
+++ b/core/src/main/scala/org/apache/spark/util/hash/PrimitiveKeyOpenHashMap.scala
@@ -28,7 +28,7 @@ package org.apache.spark.util.hash
 private[spark]
 class PrimitiveKeyOpenHashMap[@specialized(Long, Int) K: ClassManifest,
                               @specialized(Long, Int, Double) V: ClassManifest](
-    var keySet: OpenHashSet[K], var _values: Array[V])
+    val keySet: OpenHashSet[K], var _values: Array[V])
   extends Iterable[(K, V)]
   with Serializable {