diff --git a/src/h1_htx.c b/src/h1_htx.c
index c5b9dd81d835..768d0f06b176 100644
--- a/src/h1_htx.c
+++ b/src/h1_htx.c
@@ -944,6 +944,7 @@ int h1_parse_msg_tlrs(struct h1m *h1m, struct htx *dsthtx,
 
 /* Appends the H1 representation of the request line <sl> to the chunk <chk>. It
  * returns 1 if data are successfully appended, otherwise it returns 0.
+ * <chk> buffer must not wrap.
  */
 int h1_format_htx_reqline(const struct htx_sl *sl, struct buffer *chk)
 {
@@ -978,6 +979,7 @@ int h1_format_htx_reqline(const struct htx_sl *sl, struct buffer *chk)
 
 /* Appends the H1 representation of the status line <sl> to the chunk <chk>. It
  * returns 1 if data are successfully appended, otherwise it returns 0.
+ * <chk> buffer must not wrap.
  */
 int h1_format_htx_stline(const struct htx_sl *sl, struct buffer *chk)
 {
@@ -1011,6 +1013,7 @@ int h1_format_htx_stline(const struct htx_sl *sl, struct buffer *chk)
 /* Appends the H1 representation of the header <n> with the value <v> to the
  * chunk <chk>. It returns 1 if data are successfully appended, otherwise it
  * returns 0.
+ * <chk> buffer must not wrap.
  */
 int h1_format_htx_hdr(const struct ist n, const struct ist v, struct buffer *chk)
 {
@@ -1035,6 +1038,7 @@ int h1_format_htx_hdr(const struct ist n, const struct ist v, struct buffer *chk
 /* Appends the H1 representation of the data <data> to the chunk <chk>. If
  * <chunked> is non-zero, it emits HTTP/1 chunk-encoded data. It returns 1 if
  * data are successfully appended, otherwise it returns 0.
+ * <chk> buffer must not wrap.
  */
 int h1_format_htx_data(const struct ist data, struct buffer *chk, int chunked)
 {
diff --git a/src/mux_h1.c b/src/mux_h1.c
index 9a0f21a340f5..fbe3bc227895 100644
--- a/src/mux_h1.c
+++ b/src/mux_h1.c
@@ -2177,6 +2177,7 @@ static size_t h1_make_reqline(struct h1s *h1s, struct h1m *h1m, struct htx *htx,
 {
 	struct h1c *h1c = h1s->h1c;
         struct htx_blk *blk;
+	struct buffer outbuf;
 	struct htx_sl *sl;
 	enum htx_blk_type type;
 	uint32_t sz;
@@ -2201,11 +2202,14 @@ static size_t h1_make_reqline(struct h1s *h1s, struct h1m *h1m, struct htx *htx,
 
 	if (b_space_wraps(&h1c->obuf))
 		b_slow_realign(&h1c->obuf, trash.area, b_data(&h1c->obuf));
+	outbuf = b_make(b_tail(&h1c->obuf), b_contig_space(&h1c->obuf), 0, 0);
 
 	sl = htx_get_blk_ptr(htx, blk);
-	if (!h1_format_htx_reqline(sl, &h1c->obuf))
+	if (!h1_format_htx_reqline(sl, &outbuf))
 		goto full;
 
+	b_add(&h1c->obuf, outbuf.data);
+
 	h1s->meth = sl->info.req.meth;
 	h1_parse_req_vsn(h1m, sl);
 
@@ -2260,6 +2264,7 @@ static size_t h1_make_stline(struct h1s *h1s, struct h1m *h1m, struct htx *htx,
 {
 	struct h1c *h1c = h1s->h1c;
 	struct htx_blk *blk;
+	struct buffer outbuf;
 	struct htx_sl *sl;
 	enum htx_blk_type type;
 	uint32_t sz;
@@ -2286,11 +2291,14 @@ static size_t h1_make_stline(struct h1s *h1s, struct h1m *h1m, struct htx *htx,
 
 	if (b_space_wraps(&h1c->obuf))
 		b_slow_realign(&h1c->obuf, trash.area, b_data(&h1c->obuf));
+	outbuf = b_make(b_tail(&h1c->obuf), b_contig_space(&h1c->obuf), 0, 0);
 
 	sl = htx_get_blk_ptr(htx, blk);
-	if (!h1_format_htx_stline(sl, &h1c->obuf))
+	if (!h1_format_htx_stline(sl, &outbuf))
 		goto full;
 
+	b_add(&h1c->obuf, outbuf.data);
+
 	h1s->status = sl->info.res.status;
 	h1_parse_res_vsn(h1m, sl);