diff --git a/opm/core/simulator/WellState.hpp b/opm/core/simulator/WellState.hpp
index baacaa179..a909371e2 100644
--- a/opm/core/simulator/WellState.hpp
+++ b/opm/core/simulator/WellState.hpp
@@ -34,7 +34,7 @@
 
 namespace Opm
 {
-    class PhaseUsage;
+    struct PhaseUsage;
 
     /// The state of a set of wells.
     class WellState