diff --git a/cpp/NonlinearConstraint-inl.h b/cpp/NonlinearConstraint-inl.h
index c02ba2a0c..0b59f6c19 100644
--- a/cpp/NonlinearConstraint-inl.h
+++ b/cpp/NonlinearConstraint-inl.h
@@ -26,7 +26,9 @@ NonlinearConstraint<Config>::NonlinearConstraint(const LagrangeKey& lagrange_key
 					size_t dim_lagrange,
 					bool isEquality) :
 	NonlinearFactor<Config>(noiseModel::Constrained::All(dim_lagrange)),
-	lagrange_key_(lagrange_key), p_(dim_lagrange), isEquality_(isEquality) {}
+	lagrange_key_(lagrange_key), p_(dim_lagrange), isEquality_(isEquality) {
+	this->keys_.push_back(lagrange_key_);
+}
 
 /* ************************************************************************* */
 template <class Config>
@@ -49,6 +51,7 @@ NonlinearConstraint1<Config, Key, X>::NonlinearConstraint1(
 				NonlinearConstraint<Config>(lagrange_key, dim_constraint, isEquality),
 				g_(boost::bind(g, _1)), G_(boost::bind(gradG, _1)), key_(key)
 {
+	this->keys_.push_back(key);
 }
 
 /* ************************************************************************* */
@@ -63,6 +66,7 @@ NonlinearConstraint1<Config, Key, X>::NonlinearConstraint1(
 				NonlinearConstraint<Config>(lagrange_key, dim_constraint, isEquality),
 				g_(g), G_(gradG), key_(key)
 {
+	this->keys_.push_back(key);
 }
 
 /* ************************************************************************* */
@@ -143,6 +147,8 @@ NonlinearConstraint2<Config, Key1, X1, Key2, X2>::NonlinearConstraint2(
 			g_(boost::bind(g, _1)), G1_(boost::bind(G1, _1)), G2_(boost::bind(G2, _1)),
 			key1_(key1), key2_(key2)
 {
+	this->keys_.push_back(key1);
+	this->keys_.push_back(key2);
 }
 
 /* ************************************************************************* */
@@ -160,6 +166,8 @@ NonlinearConstraint2<Config, Key1, X1, Key2, X2>::NonlinearConstraint2(
 				g_(g), G1_(G1), G2_(G2),
 				key1_(key1), key2_(key2)
 {
+	this->keys_.push_back(key1);
+	this->keys_.push_back(key2);
 }
 
 /* ************************************************************************* */