diff --git a/dumux/material/fluidsystems/h2on2.hh b/dumux/material/fluidsystems/h2on2.hh
index c84572b622fa49d23a5ea74bc64ba753740a100c..e3c0d40652cf9a6ace600b00cfd43314fa66cd38 100644
--- a/dumux/material/fluidsystems/h2on2.hh
+++ b/dumux/material/fluidsystems/h2on2.hh
@@ -249,7 +249,7 @@ public:
                                       const int phaseIdx,
                                       const int compIdx)
     {
-        assert(compIdx == wCompIdx && phaseIdx == liquidPhaseIdx);
+        assert(compIdx == H2OIdx && phaseIdx == liquidPhaseIdx);
 
         using std::exp;
         return fugacityCoefficient(fluidState, phaseIdx, compIdx)