diff --git a/opm/simulators/linalg/DILU.hpp b/opm/simulators/linalg/DILU.hpp
index 2874ad927..09007ce60 100644
--- a/opm/simulators/linalg/DILU.hpp
+++ b/opm/simulators/linalg/DILU.hpp
@@ -29,6 +29,7 @@
 #include <opm/simulators/linalg/GraphColoring.hpp>
 
 #include <cstddef>
+#include <optional>
 #include <vector>
 
 #if HAVE_OPENMP