diff --git a/src/mux_h1.c b/src/mux_h1.c
index 5bd9f39..0c5fc44 100644
--- a/src/mux_h1.c
+++ b/src/mux_h1.c
@@ -1466,6 +1466,14 @@
 			TRACE_USER((!(h1m->flags & H1_MF_RESP) ? "rcvd H1 request headers" : "rcvd H1 response headers"),
 				   H1_EV_RX_DATA|H1_EV_RX_HDRS, h1c->conn, h1s, htx, (size_t[]){ret});
 
+			/* Reject Protocol upgrade request with payload */
+			if ((h1m->flags & (H1_MF_RESP|H1_MF_CONN_UPG)) == H1_MF_CONN_UPG && h1m->state != H1_MSG_DONE) {
+				h1s->flags |= H1S_F_NOT_IMPL_ERROR;
+				TRACE_USER("Upgrade with body not implemented, reject H1 message",
+					   H1_EV_RX_DATA|H1_EV_RX_HDRS|H1_EV_H1S_ERR, h1s->h1c->conn, h1s);
+				break;
+			}
+
 			if ((h1m->flags & H1_MF_RESP) &&
 			    h1s->status < 200 && (h1s->status == 100 || h1s->status >= 102)) {
 				h1m_init_res(&h1s->res);