diff --git a/src/mux_h1.c b/src/mux_h1.c
index 56937391f..bdce6dc7f 100644
--- a/src/mux_h1.c
+++ b/src/mux_h1.c
@@ -1576,7 +1576,8 @@ static size_t h1_process_output(struct h1c *h1c, struct buffer *buf, size_t coun
 					}
 				}
 
-				if ((h1m->flags & (H1_MF_CLEN|H1_MF_CHNK|H1_MF_XFER_LEN)) == H1_MF_XFER_LEN) {
+				if ((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))
 						goto copy;