diff --git a/src/h3.c b/src/h3.c
index 849bba2a6..bc479ce40 100644
--- a/src/h3.c
+++ b/src/h3.c
@@ -2678,7 +2678,7 @@ static int h3_resp_data_send(struct qcs *qcs, struct htx *htx,
 	 * buffer to perform zero-copy. This is only achievable if MUX buffer
 	 * is currently empty.
 	 */
-	if (unlikely(fsize == count &&
+	if (unlikely(fsize == count && b_size(res) == b_size(buf) &&
 	             !b_data(res) &&
 	             htx_nbblks(htx) == 1 && type == HTX_BLK_DATA)) {
 		void *old_area = res->area;
diff --git a/src/mux_fcgi.c b/src/mux_fcgi.c
index 45b546f01..33153fffa 100644
--- a/src/mux_fcgi.c
+++ b/src/mux_fcgi.c
@@ -2151,7 +2151,8 @@ static size_t fcgi_strm_send_stdin(struct fcgi_conn *fconn, struct fcgi_strm *fs
 		goto end;
 	type = htx_get_blk_type(blk);
 	size = htx_get_blksz(blk);
-	if (unlikely(size == count && htx_nbblks(htx) == 1 && type == HTX_BLK_DATA)) {
+	if (unlikely(size == count && b_size(mbuf) == b_size(buf) &&
+		     htx_nbblks(htx) == 1 && type == HTX_BLK_DATA)) {
 		void *old_area = mbuf->area;
 		int eom = (htx->flags & HTX_FL_EOM);
 
diff --git a/src/mux_h1.c b/src/mux_h1.c
index 045fd603a..bdb461237 100644
--- a/src/mux_h1.c
+++ b/src/mux_h1.c
@@ -3058,7 +3058,8 @@ static size_t h1_make_data(struct h1s *h1s, struct h1m *h1m, struct buffer *buf,
 	    (!(h1m->flags & H1_MF_CHNK) || ((h1m->flags & H1_MF_CHNK) && (!h1m->curr_len || count == h1m->curr_len))) &&
 	    htx_nbblks(htx) == 1 &&
 	    htx_get_blk_type(blk) == HTX_BLK_DATA &&
-	    htx_get_blk_value(htx, blk).len == count) {
+	    htx_get_blk_value(htx, blk).len == count &&
+	    b_size(&h1c->obuf) == b_size(buf)) {
 		void *old_area;
 		int eom = (htx->flags & HTX_FL_EOM);
 
@@ -3344,7 +3345,8 @@ static size_t h1_make_tunnel(struct h1s *h1s, struct h1m *h1m, struct buffer *bu
 	if (!b_data(&h1c->obuf) &&
 	    htx_nbblks(htx) == 1 &&
 	    htx_get_blk_type(blk) == HTX_BLK_DATA &&
-	    htx_get_blksz(blk) == count) {
+	    htx_get_blksz(blk) == count &&
+	    b_size(&h1c->obuf) == b_size(buf)) {
 		void *old_area;
 
 		old_area = h1c->obuf.area;
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 41fbbc26d..c2fb15efc 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -7234,7 +7234,7 @@ static size_t h2s_make_data(struct h2s *h2s, struct buffer *buf, size_t count)
 	 * the time. This goes for headers, data blocks and any data extracted
 	 * from the HTX blocks.
 	 */
-	if (unlikely(fsize == count &&
+	if (unlikely(fsize == count && b_size(mbuf) == b_size(buf) &&
 	             htx_nbblks(htx) == 1 && type == HTX_BLK_DATA &&
 	             fsize <= h2s_mws(h2s) && fsize <= h2c->mws && fsize <= h2c->mfs)) {
 		void *old_area = mbuf->area;