diff --git a/opm/core/linalg/ParallelIstlInformation.hpp b/opm/core/linalg/ParallelIstlInformation.hpp
index 80c2a6fc3..0aad8a1a5 100644
--- a/opm/core/linalg/ParallelIstlInformation.hpp
+++ b/opm/core/linalg/ParallelIstlInformation.hpp
@@ -345,7 +345,7 @@ private:
         template<class T, class T1>
         T operator()(const T& t1, const T& t2, const T1& mask)
         {
-            b_(t1, maskValue(t2, mask));
+            return b_(t1, maskValue(t2, mask));
         }
         template<class T, class T1>
         T maskValue(const T& t, const T1& mask)
@@ -377,7 +377,7 @@ private:
         template<class T, class T1>
         T operator()(const T& t1, const T& t2, const T1& mask)
         {
-            b_(t1, maskValue(t2, mask));
+            return b_(t1, maskValue(t2, mask));
         }
         template<class T, class T1>
         T maskValue(const T& t, const T1& mask)