diff --git a/core/api/src/main/java/org/onlab/onos/net/intent/constraint/AnnotationConstraint.java b/core/api/src/main/java/org/onlab/onos/net/intent/constraint/AnnotationConstraint.java
index ac76303..59b5ad2 100644
--- a/core/api/src/main/java/org/onlab/onos/net/intent/constraint/AnnotationConstraint.java
+++ b/core/api/src/main/java/org/onlab/onos/net/intent/constraint/AnnotationConstraint.java
@@ -41,6 +41,12 @@
         this.threshold = threshold;
     }
 
+    // Constructor for serialization
+    private AnnotationConstraint() {
+        this.key = "";
+        this.threshold = 0;
+    }
+
     /**
      * Returns the key of link annotation this constraint designates.
      * @return key of link annotation