diff --git a/src/mux_h1.c b/src/mux_h1.c
index a20e29a32..32004c68c 100644
--- a/src/mux_h1.c
+++ b/src/mux_h1.c
@@ -1404,7 +1404,7 @@ static size_t h1_process_output(struct h1c *h1c, struct buffer *buf, size_t coun
 	 * the HTX blocks.
 	 */
 	if (!b_data(&h1c->obuf)) {
-		h1c->obuf.head = sizeof(struct htx);
+		h1c->obuf.head = sizeof(struct htx) + blk->addr;
 
 		if (chn_htx->used == 1 &&
 		    blk && htx_get_blk_type(blk) == HTX_BLK_DATA &&
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 4ca0907b6..d13bd9582 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -4335,7 +4335,7 @@ static size_t h2s_htx_frt_make_resp_data(struct h2s *h2s, struct buffer *buf, si
 		 * frame header there.
 		 */
 		h2c->mbuf.area = buf->area;
-		h2c->mbuf.head = sizeof(struct htx) - 9;
+		h2c->mbuf.head = sizeof(struct htx) + blk->addr - 9;
 		h2c->mbuf.data = fsize + 9;
 		outbuf.area    = b_head(&h2c->mbuf);