diff --git a/src/main/java/net/onrc/onos/ofcontroller/util/Dpid.java b/src/main/java/net/onrc/onos/ofcontroller/util/Dpid.java
index bd91daa..81223d2 100644
--- a/src/main/java/net/onrc/onos/ofcontroller/util/Dpid.java
+++ b/src/main/java/net/onrc/onos/ofcontroller/util/Dpid.java
@@ -13,7 +13,7 @@
 @JsonDeserialize(using=DpidDeserializer.class)
 @JsonSerialize(using=DpidSerializer.class)
 public class Dpid {
-    static public long UNKNOWN = 0;
+    static public final long UNKNOWN = 0;
 
     private long value;