diff --git a/ApplicationLibCode/ProjectDataModel/RimSummaryCalculation.h b/ApplicationLibCode/ProjectDataModel/RimSummaryCalculation.h
index 96448b376d..e307e1420d 100644
--- a/ApplicationLibCode/ProjectDataModel/RimSummaryCalculation.h
+++ b/ApplicationLibCode/ProjectDataModel/RimSummaryCalculation.h
@@ -21,6 +21,8 @@
 #include "RimSummaryCalculationVariable.h"
 #include "RimUserDefinedCalculation.h"
 
+#include <optional>
+
 //==================================================================================================
 ///
 ///