diff --git a/gtsam_unstable/slam/Mechanization_bRn2.cpp b/gtsam_unstable/slam/Mechanization_bRn2.cpp
index edd92b580..94126a68c 100644
--- a/gtsam_unstable/slam/Mechanization_bRn2.cpp
+++ b/gtsam_unstable/slam/Mechanization_bRn2.cpp
@@ -66,14 +66,14 @@ Mechanization_bRn2 Mechanization_bRn2::initialize(const Matrix& U,
 }
 
 /* ************************************************************************* */
-Mechanization_bRn2 Mechanization_bRn2::correct(const Vector3& dx) const {
-  Vector3 rho = dx.segment(0,3);
+Mechanization_bRn2 Mechanization_bRn2::correct(const Vector9& dx) const {
+  Vector3 rho = dx.segment<3>(0);
 
   Rot3 delta_nRn = Rot3::Rodrigues(rho);
   Rot3 bRn = bRn_ * delta_nRn;
 
-  Vector3 x_g = x_g_ + dx.segment(3,3);
-  Vector3 x_a = x_a_ + dx.segment(6,3);
+  Vector3 x_g = x_g_ + dx.segment<3>(3);
+  Vector3 x_a = x_a_ + dx.segment<3>(6);
 
   return Mechanization_bRn2(bRn, x_g, x_a);
 }
diff --git a/gtsam_unstable/slam/Mechanization_bRn2.h b/gtsam_unstable/slam/Mechanization_bRn2.h
index 4c85614d4..4268aa0e5 100644
--- a/gtsam_unstable/slam/Mechanization_bRn2.h
+++ b/gtsam_unstable/slam/Mechanization_bRn2.h
@@ -68,7 +68,7 @@ public:
    * @param obj The current state
    * @param dx The error used to correct and return a new state
    */
-  Mechanization_bRn2 correct(const Vector3& dx) const;
+  Mechanization_bRn2 correct(const Vector9& dx) const;
 
   /**
    * Integrate to get new state