diff --git a/lib/libf2c/Makefile b/lib/libf2c/Makefile
index 5db8a42817f..e05413008b9 100644
--- a/lib/libf2c/Makefile
+++ b/lib/libf2c/Makefile
@@ -1,7 +1,7 @@
 .PATH:	${.CURDIR}/../libF77 ${.CURDIR}/../libI77
 
 LIB=f2c
-CFLAGS+= -DIEEE_drem -DNON_ANSI_RW_MODES -DNON_UNIX_STDIO -DPedantic
+CFLAGS+= -DIEEE_drem -DNON_ANSI_RW_MODES -DNON_UNIX_STDIO -DPedantic -I${.CURDIR}/../../usr.bin/f2c
 
 MISC =	Version.c main.c s_rnge.c abort_.c getarg_.c iargc_.c getenv_.c\
 	signal_.c s_stop.c s_paus.c system_.c cabs.c\