diff --git a/gtsam/linear/NoiseModel.h b/gtsam/linear/NoiseModel.h
index a4d15c0f8..73f663240 100644
--- a/gtsam/linear/NoiseModel.h
+++ b/gtsam/linear/NoiseModel.h
@@ -219,9 +219,7 @@ namespace gtsam {
       }
 
 #ifdef GTSAM_ALLOW_DEPRECATED_SINCE_V4
-      virtual double Mahalanobis(const Vector& v) const {
-        return SquaredMahalanobisDistance(v);
-      }
+      virtual double Mahalanobis(const Vector& v) const;
 #endif
 
       inline virtual double distance(const Vector& v) const {