diff --git a/ProcessLib/SurfaceFlux/SurfaceFluxLocalAssembler.h b/ProcessLib/SurfaceFlux/SurfaceFluxLocalAssembler.h
index 69aa70dbf5823995f2e28ec6f717d4bb45f5d8bb..2347aaed24ab6f22f2ba8f942f3a0d2fcc62a088 100644
--- a/ProcessLib/SurfaceFlux/SurfaceFluxLocalAssembler.h
+++ b/ProcessLib/SurfaceFlux/SurfaceFluxLocalAssembler.h
@@ -181,8 +181,8 @@ private:
     std::vector<double> _detJ_times_integralMeasure;
 
     IntegrationMethod const _integration_method;
-    std::size_t _bulk_element_id;
-    std::size_t _bulk_face_id;
+    std::size_t const _bulk_element_id;
+    std::size_t const _bulk_face_id;
 };
 
 }  // namespace ProcessLib