diff --git a/gtsam/navigation/MagFactor.h b/gtsam/navigation/MagFactor.h
index 96a0971c5..9abd70e58 100644
--- a/gtsam/navigation/MagFactor.h
+++ b/gtsam/navigation/MagFactor.h
@@ -189,7 +189,7 @@ public:
    * @param nM (unknown) local earth magnetic field vector, in nav frame
    * @param bias (unknown) 3D bias
    */
-  Vector evaluateError(double scale, const Unit3& direction,
+  Vector evaluateError(const double& scale, const Unit3& direction,
       const Point3& bias, boost::optional<Matrix&> H1 = boost::none,
       boost::optional<Matrix&> H2 = boost::none, boost::optional<Matrix&> H3 =
           boost::none) const {
diff --git a/gtsam/navigation/tests/testMagFactor.cpp b/gtsam/navigation/tests/testMagFactor.cpp
index 5599c93d6..80f67c58d 100644
--- a/gtsam/navigation/tests/testMagFactor.cpp
+++ b/gtsam/navigation/tests/testMagFactor.cpp
@@ -48,7 +48,7 @@ Point3 bias(10, -10, 50);
 Point3 scaled = scale * nM;
 Point3 measured = nRb.inverse() * (scale * nM) + bias;
 
-LieScalar s(scale * nM.norm());
+double s(scale * nM.norm());
 Unit3 dir(nM);
 
 SharedNoiseModel model = noiseModel::Isotropic::Sigma(3, 0.25);
@@ -94,7 +94,7 @@ TEST( MagFactor, Factors ) {
 // MagFactor2
   MagFactor3 f3(1, 2, 3, measured, nRb, model);
   EXPECT(assert_equal(zero(3),f3.evaluateError(s,dir,bias,H1,H2,H3),1e-5));
-  EXPECT(assert_equal(numericalDerivative11<Vector,LieScalar> //
+  EXPECT(assert_equal(numericalDerivative11<Vector,double> //
       (boost::bind(&MagFactor3::evaluateError, &f3, _1, dir, bias, none, none, none), s),//
       H1, 1e-7));
   EXPECT(assert_equal(numericalDerivative11<Vector,Unit3> //