diff --git a/source/blender/blenkernel/intern/modifier.c b/source/blender/blenkernel/intern/modifier.c
index 0cd6035b9dc..ca95d9edc06 100644
--- a/source/blender/blenkernel/intern/modifier.c
+++ b/source/blender/blenkernel/intern/modifier.c
@@ -1506,7 +1506,8 @@ static DerivedMesh *mirrorModifier_applyModifier(
 
 	result = mirrorModifier__doMirror(mmd, ob, derivedData, 0);
 
-	CDDM_calc_normals(result);
+	if(result != derivedData)
+		CDDM_calc_normals(result);
 	
 	return result;
 }