diff --git a/gtsam/slam/simulated3D.h b/gtsam/slam/simulated3D.h
index a386736a1..7b4dfce37 100644
--- a/gtsam/slam/simulated3D.h
+++ b/gtsam/slam/simulated3D.h
@@ -86,7 +86,7 @@ struct PointPrior3D: public NoiseModelFactor1<Point3> {
 	 * @return Vector error between prior value and x (Dimension: 3)
 	 */
 	Vector evaluateError(const Point3& x, boost::optional<Matrix&> H =
-			boost::none) {
+			boost::none) const {
 		return (prior(x, H) - measured_).vector();
 	}
 };
@@ -118,7 +118,7 @@ struct Simulated3DMeasurement: public NoiseModelFactor2<Point3, Point3> {
 	 * @return vector error between measurement and prediction (Dimension: 3)
 	 */
 	Vector evaluateError(const Point3& x1, const Point3& x2,
-	    boost::optional<Matrix&> H1 = boost::none, boost::optional<Matrix&> H2 = boost::none) {
+	    boost::optional<Matrix&> H1 = boost::none, boost::optional<Matrix&> H2 = boost::none) const {
 		return (mea(x1, x2, H1, H2) - measured_).vector();
 	}
 };