diff --git a/hyracks-control-cc/src/main/java/edu/uci/ics/hyracks/control/cc/scheduler/JobScheduler.java b/hyracks-control-cc/src/main/java/edu/uci/ics/hyracks/control/cc/scheduler/JobScheduler.java
index 6933bf0..24bee8c 100644
--- a/hyracks-control-cc/src/main/java/edu/uci/ics/hyracks/control/cc/scheduler/JobScheduler.java
+++ b/hyracks-control-cc/src/main/java/edu/uci/ics/hyracks/control/cc/scheduler/JobScheduler.java
@@ -461,10 +461,11 @@
     }
 
     private void abortJob(Exception exception) {
-        for (TaskCluster tc : inProgressTaskClusters) {
+        Set<TaskCluster> inProgressTaskClustersCopy = new HashSet<TaskCluster>(inProgressTaskClusters);
+        for (TaskCluster tc : inProgressTaskClustersCopy) {
             abortTaskCluster(findLastTaskClusterAttempt(tc));
         }
-        inProgressTaskClusters.clear();
+        assert inProgressTaskClusters.isEmpty();
         ccs.getWorkQueue().schedule(new JobCleanupWork(ccs, jobRun.getJobId(), JobStatus.FAILURE, exception));
     }