diff --git a/dumux/material/fluidsystems/brineco2.hh b/dumux/material/fluidsystems/brineco2.hh
index 6adc767dc064908cc9c7406b4ae14650a48de3e1..800cecd550a9a7e0d011cd105ab8c53c661629fc 100644
--- a/dumux/material/fluidsystems/brineco2.hh
+++ b/dumux/material/fluidsystems/brineco2.hh
@@ -250,8 +250,8 @@ public:
     static bool isIdealMixture(int phaseIdx)
     {
         assert(0 <= phaseIdx && phaseIdx < numPhases);
-        if (!useConstantSalinity && phaseIdx == liquidPhaseIdx)
-            return VariableSalinityBrine::isIdealMixture(VariableSalinityBrine::liquidPhaseIdx);
+        if (phaseIdx == liquidPhaseIdx)
+            return false;
         return true;
     }
 
diff --git a/test/material/fluidsystems/checkfluidsystem.hh b/test/material/fluidsystems/checkfluidsystem.hh
index 857ed169829e9a099c955e90906dbaa2b4e15297..3e972f2fefb8b4be6eabd6f494b737dda9ada7d4 100644
--- a/test/material/fluidsystems/checkfluidsystem.hh
+++ b/test/material/fluidsystems/checkfluidsystem.hh
@@ -89,10 +89,7 @@ public:
             BaseFluidState::setMolarDensity(phaseIdx, 1.0);
 
             for (int compIdx = 0; compIdx < numComponents; ++compIdx)
-            {
                 BaseFluidState::setMoleFraction(phaseIdx, compIdx, 1.0 / numComponents);
-
-            }
         }
 
         // initially, do not allow anything
@@ -398,8 +395,16 @@ int checkFluidState(const BaseFluidState &fs)
     }
 }
 
+/*!
+ * \brief This is a consistency check for FluidSystems.
+ * \param enablePhaseRestriction Parameter passed to the fluidState. If set to true,
+ *        the fluidState will only allow calls to properties of the current phase.
+ * \note While this is very common, it is not necessarily the case for all FluidSystems.
+ *       We keep this, because it might help finding mistakes in FluidSystems that have this invariant.
+ *       If you verified that a fluid system does not have this invariant you can set this option to false.
+ */
 template<class Scalar, class FluidSystem>
-int checkFluidSystem()
+int checkFluidSystem(bool enablePhaseRestriction = true)
 {
     int success = 0;
     std::cout << "Testing fluid system '" << Dune::className<FluidSystem>() << "'\n";
@@ -460,7 +465,8 @@ int checkFluidSystem()
 
     for (int phaseIdx = 0; phaseIdx < numPhases; ++phaseIdx)
     {
-        fs.restrictToPhase(phaseIdx);
+        if (enablePhaseRestriction)
+            fs.restrictToPhase(phaseIdx);
         try
         {
             paramCache.updatePhase(fs, phaseIdx);
@@ -519,10 +525,11 @@ int checkFluidSystem()
     // actually check the fluid system API
     for (int phaseIdx = 0; phaseIdx < numPhases; ++phaseIdx)
     {
-        fs.restrictToPhase(phaseIdx);
         fs.allowPressure(FluidSystem::isCompressible(phaseIdx));
         fs.allowComposition(true);
         fs.allowDensity(false);
+        if (enablePhaseRestriction)
+            fs.restrictToPhase(phaseIdx);
         try
         {
             val = FluidSystem::density(fs, paramCache, phaseIdx);
diff --git a/test/material/fluidsystems/test_fluidsystems.cc b/test/material/fluidsystems/test_fluidsystems.cc
index 4befb937b0c47dad1d62991f35c06920e55b61f9..5ce0d7d5f4b67861b929d8c786880c46f87acbf8 100644
--- a/test/material/fluidsystems/test_fluidsystems.cc
+++ b/test/material/fluidsystems/test_fluidsystems.cc
@@ -149,30 +149,34 @@ int main()
         success += checkFluidSystem<Scalar, FluidSystem>(); }
 
     // Brine -- CO2
+    // BrineCO2 does not fulfill the restrictToPhase-assertion where we assume that for all
+    // functions depending on a phase index only fluid properties of this phase are used
+    // that is why checkFluidSystem() needs to be called with "false" here
+    // see checkFLuidSystem documentation!
     {   using H2OType = Components::SimpleH2O<Scalar>;
         using FluidSystem = FluidSystems::BrineCO2< Scalar, HeterogeneousCO2Tables::CO2Tables,
                                                     H2OType, FluidSystems::BrineCO2DefaultPolicy</*useConstantSalinity=*/true> >;
-        success += checkFluidSystem<Scalar, FluidSystem>(); }
+        success += checkFluidSystem<Scalar, FluidSystem>( false ); }
     {   using H2OType = Components::SimpleH2O<Scalar>;
         using FluidSystem = FluidSystems::BrineCO2< Scalar, HeterogeneousCO2Tables::CO2Tables,
                                                     H2OType, FluidSystems::BrineCO2DefaultPolicy</*useConstantSalinity=*/false> >;
-        success += checkFluidSystem<Scalar, FluidSystem>(); }
+        success += checkFluidSystem<Scalar, FluidSystem>( false ); }
     {   using H2OType = Components::H2O<Scalar>;
         using FluidSystem = FluidSystems::BrineCO2< Scalar, HeterogeneousCO2Tables::CO2Tables,
                                                     H2OType, FluidSystems::BrineCO2DefaultPolicy</*useConstantSalinity=*/true> >;
-        success += checkFluidSystem<Scalar, FluidSystem>(); }
+        success += checkFluidSystem<Scalar, FluidSystem>( false ); }
     {   using H2OType = Components::H2O<Scalar>;
         using FluidSystem = FluidSystems::BrineCO2< Scalar, HeterogeneousCO2Tables::CO2Tables,
                                                     H2OType, FluidSystems::BrineCO2DefaultPolicy</*useConstantSalinity=*/false> >;
-        success += checkFluidSystem<Scalar, FluidSystem>(); }
+        success += checkFluidSystem<Scalar, FluidSystem>( false ); }
     {   using H2OType = Components::TabulatedComponent<Components::H2O<Scalar>>;
         using FluidSystem = FluidSystems::BrineCO2< Scalar, HeterogeneousCO2Tables::CO2Tables,
                                                     H2OType, FluidSystems::BrineCO2DefaultPolicy</*useConstantSalinity=*/true> >;
-        success += checkFluidSystem<Scalar, FluidSystem>(); }
+        success += checkFluidSystem<Scalar, FluidSystem>( false ); }
     {   using H2OType = Components::TabulatedComponent<Components::H2O<Scalar>>;
         using FluidSystem = FluidSystems::BrineCO2< Scalar, HeterogeneousCO2Tables::CO2Tables,
                                                     H2OType, FluidSystems::BrineCO2DefaultPolicy</*useConstantSalinity=*/false> >;
-        success += checkFluidSystem<Scalar, FluidSystem>(); }
+        success += checkFluidSystem<Scalar, FluidSystem>( false ); }
 
     // H2O -- Air
     {   using H2OType = Components::SimpleH2O<Scalar>;