diff --git a/gtsam/slam/PoseRotationPrior.h b/gtsam/slam/PoseRotationPrior.h
index ba4d12a25d..20f12dbce2 100644
--- a/gtsam/slam/PoseRotationPrior.h
+++ b/gtsam/slam/PoseRotationPrior.h
@@ -39,6 +39,9 @@ class PoseRotationPrior : public NoiseModelFactor1<POSE> {
 
 public:
 
+  /** default constructor - only use for serialization */
+  PoseRotationPrior() {}
+
   /** standard constructor */
   PoseRotationPrior(Key key, const Rotation& rot_z, const SharedNoiseModel& model)
   : Base(model, key), measured_(rot_z) {}