diff --git a/opm/output/eclipse/WriteRPT.hpp b/opm/output/eclipse/WriteRPT.hpp
index 8f62f70c4..67666d814 100644
--- a/opm/output/eclipse/WriteRPT.hpp
+++ b/opm/output/eclipse/WriteRPT.hpp
@@ -20,6 +20,7 @@
 #ifndef OPM_WRITE_RPT_HPP
 #define OPM_WRITE_RPT_HPP
 
+#include <cstddef>
 #include <iosfwd>
 
 namespace Opm {