diff --git a/opm/simulators/wells/WGState.hpp b/opm/simulators/wells/WGState.hpp
index d0cd41412..555b513b6 100644
--- a/opm/simulators/wells/WGState.hpp
+++ b/opm/simulators/wells/WGState.hpp
@@ -29,7 +29,7 @@ namespace Opm {
   Microscopic class to handle a pair of well and group state.
 */
 
-class PhaseUsage;
+struct PhaseUsage;
 
 struct WGState {
     WGState(const PhaseUsage& pu);