diff --git a/dumux/material/fluidsystems/h2o_n2_system.hh b/dumux/material/fluidsystems/h2o_n2_system.hh
index 4329d2ae9687e51cd5baf81692e81f4b9b20bd79..9ee343376643552413b2813cb6d3f742702aca0a 100644
--- a/dumux/material/fluidsystems/h2o_n2_system.hh
+++ b/dumux/material/fluidsystems/h2o_n2_system.hh
@@ -184,6 +184,7 @@ public:
                                         pressure);
         }
         else {
+#warning "TODO: Viscosity of gas phase does not depend on composition!"
             return N2::gasViscosity(temperature,
                                     pressure);
             /* Wilke method. See: