diff --git a/thresh/src/main/java/monasca/thresh/infrastructure/thresholding/MetricAggregationBolt.java b/thresh/src/main/java/monasca/thresh/infrastructure/thresholding/MetricAggregationBolt.java
index 2ec9a73..fe7a47a 100644
--- a/thresh/src/main/java/monasca/thresh/infrastructure/thresholding/MetricAggregationBolt.java
+++ b/thresh/src/main/java/monasca/thresh/infrastructure/thresholding/MetricAggregationBolt.java
@@ -219,8 +219,24 @@ public class MetricAggregationBolt extends BaseRichBolt {
 
   private void sendSubAlarmStateChange(SubAlarmStats subAlarmStats) {
     logger.debug("Alarm state changed for {}", subAlarmStats);
-    collector.emit(new Values(subAlarmStats.getSubAlarm().getAlarmId(), subAlarmStats
-        .getSubAlarm()));
+    collector.emit(new Values(subAlarmStats.getSubAlarm().getAlarmId(), duplicate(subAlarmStats
+        .getSubAlarm())));
+  }
+
+  /**
+   * Create a copy of SubAlarm to prevent ConcurrentModificationExceptions thrown by Storm.
+   * The AlarmSubExpression is not immutable, but since it is only replaced in this Bolt,
+   * it can be reused
+   * @param original SubAlarm to be duplicated
+   * @return copy of original
+   */
+  public SubAlarm duplicate(final SubAlarm original) {
+    final SubAlarm newSubAlarm =
+        new SubAlarm(original.getId(), original.getAlarmId(), new SubExpression(
+            original.getAlarmSubExpressionId(), original.getExpression()), original.getState());
+    newSubAlarm.setNoState(original.isNoState());
+    newSubAlarm.setSporadicMetric(original.isSporadicMetric());
+    return newSubAlarm;
   }
 
   /**