diff --git a/opm/simulators/flow/ExtboContainer.hpp b/opm/simulators/flow/ExtboContainer.hpp
index 19ce826de..20b5edab5 100644
--- a/opm/simulators/flow/ExtboContainer.hpp
+++ b/opm/simulators/flow/ExtboContainer.hpp
@@ -57,6 +57,7 @@ public:
     bool allocated() const
     { return allocated_; }
 
+private:
     bool allocated_ = false;
     ScalarBuffer X_volume_;
     ScalarBuffer Y_volume_;