diff --git a/gtsam/nonlinear/nonlinear.i b/gtsam/nonlinear/nonlinear.i
index 6d101af98..a24bcbb75 100644
--- a/gtsam/nonlinear/nonlinear.i
+++ b/gtsam/nonlinear/nonlinear.i
@@ -507,18 +507,18 @@ class ISAM2 {
   gtsam::ISAM2Result update(const gtsam::NonlinearFactorGraph& newFactors,
                             const gtsam::Values& newTheta,
                             const gtsam::FactorIndices& removeFactorIndices,
-                            gtsam::KeyGroupMap& constrainedKeys,
+                            const gtsam::KeyGroupMap& constrainedKeys,
                             const gtsam::KeyList& noRelinKeys);
   gtsam::ISAM2Result update(const gtsam::NonlinearFactorGraph& newFactors,
                             const gtsam::Values& newTheta,
                             const gtsam::FactorIndices& removeFactorIndices,
-                            gtsam::KeyGroupMap& constrainedKeys,
+                            const gtsam::KeyGroupMap& constrainedKeys,
                             const gtsam::KeyList& noRelinKeys,
                             const gtsam::KeyList& extraReelimKeys);
   gtsam::ISAM2Result update(const gtsam::NonlinearFactorGraph& newFactors,
                             const gtsam::Values& newTheta,
                             const gtsam::FactorIndices& removeFactorIndices,
-                            gtsam::KeyGroupMap& constrainedKeys,
+                            const gtsam::KeyGroupMap& constrainedKeys,
                             const gtsam::KeyList& noRelinKeys,
                             const gtsam::KeyList& extraReelimKeys,
                             bool force_relinearize);
@@ -527,6 +527,8 @@ class ISAM2 {
                             const gtsam::Values& newTheta,
                             const gtsam::ISAM2UpdateParams& updateParams);
 
+  double error(const gtsam::VectorValues& values) const;
+
   gtsam::Values getLinearizationPoint() const;
   bool valueExists(gtsam::Key key) const;
   gtsam::Values calculateEstimate() const;