diff --git a/sys/dev/md/md.c b/sys/dev/md/md.c
--- a/sys/dev/md/md.c
+++ b/sys/dev/md/md.c
@@ -147,8 +147,16 @@
 	int		md_fwsectors;
 	uint32_t	md_label;
 	int		md_pad[MDNPAD];
-} __attribute__((__packed__));
+}
+#ifdef __amd64__
+__attribute__((__packed__))
+#endif
+;
+#ifndef __amd64__
+CTASSERT((sizeof(struct md_ioctl32)) == 440);
+#else
 CTASSERT((sizeof(struct md_ioctl32)) == 436);
+#endif
 
 #define	MDIOCATTACH_32	_IOC_NEWTYPE(MDIOCATTACH, struct md_ioctl32)
 #define	MDIOCDETACH_32	_IOC_NEWTYPE(MDIOCDETACH, struct md_ioctl32)