diff --git a/src/Makefile.MESONH.mk b/src/Makefile.MESONH.mk
index 25ff4c374bbb2515246accf216c7af337c9bd995..5c10924d258d66d10c3311f1bd36a878ed092142 100644
--- a/src/Makefile.MESONH.mk
+++ b/src/Makefile.MESONH.mk
@@ -136,7 +136,7 @@ endif
 ##########################################################
 ifdef MNH_RTTOV
 DIR_RTTOV      +=  LIB/RTTOV/src
-#CPPFLAGS_RTTOV =
+CPPFLAGS_RTTOV = -DMNH_RTTOV
 INC_RTTOV      = -I$(B)LIB/RTTOV/src
 #
 DIR_MASTER  += $(DIR_RTTOV)