diff --git a/core/src/main/scala/org/apache/spark/util/collection/OpenHashMap.scala b/core/src/main/scala/org/apache/spark/util/collection/OpenHashMap.scala
index 22d7a4988bb565aa520a006b3182fc17a2b9d672..10ab0b3f89964d9ba8bc9fb7ddd62a79339a84a7 100644
--- a/core/src/main/scala/org/apache/spark/util/collection/OpenHashMap.scala
+++ b/core/src/main/scala/org/apache/spark/util/collection/OpenHashMap.scala
@@ -25,6 +25,9 @@ import scala.reflect.ClassTag
  * space overhead.
  *
  * Under the hood, it uses our OpenHashSet implementation.
+ *
+ * NOTE: when using numeric type as the value type, the user of this class should be careful to
+ * distinguish between the 0/0.0/0L and non-exist value
  */
 private[spark]
 class OpenHashMap[K : ClassTag, @specialized(Long, Int, Double) V: ClassTag](