diff --git a/drivers/tapdisk-image.c b/drivers/tapdisk-image.c
index eac7cdd1..b9732037 100644
--- a/drivers/tapdisk-image.c
+++ b/drivers/tapdisk-image.c
@@ -143,6 +143,7 @@ tapdisk_image_check_request(td_image_t *image, td_vbd_request_t *vreq)
 
 	switch (vreq->op) {
 	case TD_OP_DISCARD:
+                secs = vreq->discard_nr_sectors;
 		/* fall through */
 	case TD_OP_WRITE:
 		if (rdonly) {