diff --git a/core/src/main/scala/org/apache/spark/network/ConnectionManager.scala b/core/src/main/scala/org/apache/spark/network/ConnectionManager.scala
index 37d69a9ec4ce4a37f0c7dbf093112472808df404..e77d762bdf2214440d872cbd1226181004f63cfc 100644
--- a/core/src/main/scala/org/apache/spark/network/ConnectionManager.scala
+++ b/core/src/main/scala/org/apache/spark/network/ConnectionManager.scala
@@ -886,6 +886,7 @@ private[spark] class ConnectionManager(
   }
 
   def stop() {
+    ackTimeoutMonitor.cancel()
     selectorThread.interrupt()
     selectorThread.join()
     selector.close()