diff --git a/gtsam/linear/NoiseModel.h b/gtsam/linear/NoiseModel.h
index 55d01c92f..0351cfabd 100644
--- a/gtsam/linear/NoiseModel.h
+++ b/gtsam/linear/NoiseModel.h
@@ -873,7 +873,7 @@ namespace gtsam {
     };
     
     // Helper function
-    boost::optional<Vector> checkIfDiagonal(const Matrix M);
+    GTSAM_EXPORT boost::optional<Vector> checkIfDiagonal(const Matrix M);
 
   } // namespace noiseModel