diff --git a/certs/Makefile b/certs/Makefile
index ab6da6f4e9530e1f3eb29d935355cdedd24c3864..49f8101ccad2755c4e2022237b54d5b4cc1d439d 100644
--- a/certs/Makefile
+++ b/certs/Makefile
@@ -67,7 +67,7 @@ $(obj)/system_certificates.o: $(obj)/signing_key.x509
 
 ifdef CONFIG_PGP_PRELOAD_PUBLIC_KEYS
 ifeq ($(shell ls $(srctree)/certs/pubring.gpg 2> /dev/null), $(srctree)/certs/pubring.gpg)
-system_certificates.o += -DHAVE_PUBRING_GPG
+AFLAGS_system_certificates.o += -DHAVE_PUBRING_GPG
 $(obj)/system_certificates.o: $(srctree)/certs/pubring.gpg
 endif
 endif