diff --git a/gtsam/slam/BetweenFactor.h b/gtsam/slam/BetweenFactor.h
index 89291fac5..c183a97ca 100644
--- a/gtsam/slam/BetweenFactor.h
+++ b/gtsam/slam/BetweenFactor.h
@@ -56,7 +56,7 @@ namespace gtsam {
 
     /** Constructor */
     BetweenFactor(Key key1, Key key2, const VALUE& measured,
-        const SharedNoiseModel& model) :
+        const SharedNoiseModel& model = nullptr) :
       Base(model, key1, key2), measured_(measured) {
     }
 
diff --git a/gtsam/slam/PriorFactor.h b/gtsam/slam/PriorFactor.h
index 3c5c42ccc..ed9340ff2 100644
--- a/gtsam/slam/PriorFactor.h
+++ b/gtsam/slam/PriorFactor.h
@@ -53,7 +53,7 @@ namespace gtsam {
     virtual ~PriorFactor() {}
 
     /** Constructor */
-    PriorFactor(Key key, const VALUE& prior, const SharedNoiseModel& model) :
+    PriorFactor(Key key, const VALUE& prior, const SharedNoiseModel& model = nullptr) :
       Base(model, key), prior_(prior) {
     }