diff --git a/sys/modules/ath/Makefile b/sys/modules/ath/Makefile
index 49f7fa698720..d969fda1088c 100644
--- a/sys/modules/ath/Makefile
+++ b/sys/modules/ath/Makefile
@@ -140,7 +140,9 @@ SRCS+=	dfs_null.c
 
 CFLAGS+=  -I. -I${.CURDIR}/../../dev/ath -I${.CURDIR}/../../dev/ath/ath_hal
 
+.if !defined(KERNBUILDDIR)
 opt_ah.h:
 	echo '#define AH_SUPPORT_AR5416 1' > $@
+.endif
 
 .include <bsd.kmod.mk>