diff --git a/src/mux_h1.c b/src/mux_h1.c
index 230903b6b..60461a819 100644
--- a/src/mux_h1.c
+++ b/src/mux_h1.c
@@ -1685,7 +1685,11 @@ static size_t h1_process_output(struct h1c *h1c, struct buffer *buf, size_t coun
 				     (h1m->flags & (H1_MF_VER_11|H1_MF_RESP|H1_MF_CLEN|H1_MF_CHNK|H1_MF_XFER_LEN)) ==
 				     (H1_MF_VER_11|H1_MF_RESP|H1_MF_XFER_LEN))) {
 					/* chunking needed but header not seen */
-					if (!chunk_memcat(&tmp, "transfer-encoding: chunked\r\n", 28))
+					n = ist("transfer-encoding");
+					v = ist("chunked");
+					if (h1c->px->options2 & (PR_O2_H1_ADJ_BUGCLI|PR_O2_H1_ADJ_BUGSRV))
+						h1_adjust_case_outgoing_hdr(h1s, h1m, &n);
+					if (!htx_hdr_to_h1(n, v, &tmp))
 						goto copy;
 					TRACE_STATE("add \"Transfer-Encoding: chunked\"", H1_EV_TX_DATA|H1_EV_TX_HDRS, h1c->conn, h1s);
 					h1m->flags |= H1_MF_CHNK;