diff --git a/lib/libc/amd64/string/Makefile.inc b/lib/libc/amd64/string/Makefile.inc
index b1369841bc7..d5bb646c5c5 100644
--- a/lib/libc/amd64/string/Makefile.inc
+++ b/lib/libc/amd64/string/Makefile.inc
@@ -28,3 +28,9 @@ MDSRCS+= \
 	strspn.S \
 	timingsafe_bcmp.S \
 	timingsafe_memcmp.S
+
+.if ${MK_ASAN} != "no"
+# Disable ASAN for amd64_archlevel.c since its code is executed before the
+# sanitizer runtime can initialize itself.
+CFLAGS.amd64_archlevel.c+=	-fno-sanitize=address
+.endif