diff --git a/core/api/src/main/java/org/onlab/onos/event/AbstractEvent.java b/core/api/src/main/java/org/onlab/onos/event/AbstractEvent.java
index 0fc8eaa..49fb268 100644
--- a/core/api/src/main/java/org/onlab/onos/event/AbstractEvent.java
+++ b/core/api/src/main/java/org/onlab/onos/event/AbstractEvent.java
@@ -26,7 +26,7 @@
 
     private final long time;
     private final T type;
-    private S subject;
+    private final S subject;
 
     /**
      * Creates an event of a given type and for the specified subject and the