diff --git a/sys/dev/disk/mmcsd/mmcsd.c b/sys/dev/disk/mmcsd/mmcsd.c
--- a/sys/dev/disk/mmcsd/mmcsd.c
+++ b/sys/dev/disk/mmcsd/mmcsd.c
@@ -335,6 +335,9 @@
 	int sz = sc->disk.d_info.d_media_blksize;
 	struct buf *bp = bio->bio_buf;
 
+	if (sz <= 0)
+		return 0;
+
 	block = bio->bio_offset / sz;
 	end = block + (bp->b_bcount / sz);
 	while (block < end) {