diff --git a/lib/libsys/Makefile b/lib/libsys/Makefile
index bc96548cb9f3..b9a50ecc2d8c 100644
--- a/lib/libsys/Makefile
+++ b/lib/libsys/Makefile
@@ -41,6 +41,10 @@ PRECIOUSLIB=
 # it will be loaded during program startup.
 CFLAGS+= -ftls-model=initial-exec
 
+.ifdef CHERI_LIB_C18N
+CFLAGS+=-DCHERI_LIB_C18N
+.endif
+
 #
 # Link with static libcompiler_rt.a.
 #