diff --git a/dumux/material/fluidsystems/brineair.hh b/dumux/material/fluidsystems/brineair.hh
index 32340796fc3850f22adb5a711e334e5e97f9b6d2..e1b1db39ff23aa565f94e3dc2eedf9e966a6e2f9 100644
--- a/dumux/material/fluidsystems/brineair.hh
+++ b/dumux/material/fluidsystems/brineair.hh
@@ -132,7 +132,7 @@ public:
     static constexpr bool isGas(int phaseIdx)
     {
         assert(0 <= phaseIdx && phaseIdx < numPhases);
-        return phaseIdx == phase0Idx;
+        return phaseIdx == gasPhaseIdx;
     }
 
     /*!