diff --git a/core/src/main/scala/org/apache/spark/util/ActorLogReceive.scala b/core/src/main/scala/org/apache/spark/util/ActorLogReceive.scala
index 332d0cbb2dc0c36401b84237c61df63d503bf87b..81a7cbde01ce5f900833eb7a4061516818c48f71 100644
--- a/core/src/main/scala/org/apache/spark/util/ActorLogReceive.scala
+++ b/core/src/main/scala/org/apache/spark/util/ActorLogReceive.scala
@@ -43,7 +43,13 @@ private[spark] trait ActorLogReceive {
 
     private val _receiveWithLogging = receiveWithLogging
 
-    override def isDefinedAt(o: Any): Boolean = _receiveWithLogging.isDefinedAt(o)
+    override def isDefinedAt(o: Any): Boolean = {
+      val handled = _receiveWithLogging.isDefinedAt(o)
+      if (!handled) {
+        log.debug(s"Received unexpected actor system event: $o")
+      }
+      handled
+    }
 
     override def apply(o: Any): Unit = {
       if (log.isDebugEnabled) {