diff --git a/vtkm/rendering/Mapper.cxx b/vtkm/rendering/Mapper.cxx
index 2f645c5f2..986e7fff2 100644
--- a/vtkm/rendering/Mapper.cxx
+++ b/vtkm/rendering/Mapper.cxx
@@ -70,7 +70,8 @@ struct CompareIndices
     }
     else
     {
-      return (vtkm::Dot(Centers[i], CameraDirection) < vtkm::Dot(Centers[j], CameraDirection));
+      return (vtkm::Dot(this->Centers[i], this->CameraDirection) <
+              vtkm::Dot(this->Centers[j], this->CameraDirection));
     }
   }
 };