diff --git a/sys/dev/disk/dm/dm_target_zero.c b/sys/dev/disk/dm/dm_target_zero.c
--- a/sys/dev/disk/dm/dm_target_zero.c
+++ b/sys/dev/disk/dm/dm_target_zero.c
@@ -40,6 +40,18 @@
 static int
 dm_target_zero_strategy(dm_table_entry_t *table_en, struct buf *bp)
 {
+	/*
+	 * Only zero data buffers (READ/WRITE).  FREEBLKS (discard/TRIM)
+	 * bios have b_data == NULL -- there is no data buffer for a
+	 * discard, only an offset and length.  Calling memset on a
+	 * FREEBLKS bio would dereference NULL and panic.
+	 */
+	if (bp->b_cmd != BUF_CMD_READ && bp->b_cmd != BUF_CMD_WRITE) {
+		bp->b_resid = 0;
+		biodone(&bp->b_bio1);
+		return 0;
+	}
+
 	memset(bp->b_data, 0, bp->b_bcount);
 	bp->b_resid = 0;
 	biodone(&bp->b_bio1);