diff --git a/src/core/RpUnits.h b/src/core/RpUnits.h
index a640205..e697424 100644
--- a/src/core/RpUnits.h
+++ b/src/core/RpUnits.h
@@ -59,6 +59,7 @@ enum RP_UNITS_CONSTS {
 #include <stdlib.h>
 #include <errno.h>
 #include <math.h>
+#include <functional>
 
 #include "RpDict.h"
 #include "RpUnitsStd.h"