diff --git a/ProcessLib/ProcessVariable.h b/ProcessLib/ProcessVariable.h
index c62fe569fedc591defe5f85760891b29707a634d..afa8edfefb2d022b6d71d5f3d25bef26a7830c27 100644
--- a/ProcessLib/ProcessVariable.h
+++ b/ProcessLib/ProcessVariable.h
@@ -67,7 +67,7 @@ public:
 
     void updateDeactivatedSubdomains(double const time);
 
-    std::vector<std::size_t>& getActiveElementIDs() const
+    std::vector<std::size_t> const& getActiveElementIDs() const
     {
         return _ids_of_active_elements;
     }