diff --git a/src/proxy.c b/src/proxy.c
index 838722d1e..a5570dafa 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -1477,6 +1477,14 @@ int stream_set_backend(struct stream *s, struct proxy *be)
 				s->flags |= SF_HTX;
 			}
 		}
+		else if (IS_HTX_STRM(s) && be->mode != PR_MODE_HTTP) {
+			/* If a TCP backend is assgiend to an HTX stream, return
+			 * an error. It may happens for a new stream on a
+			 * previously upgraded connnections. */
+			if (!(s->flags & SF_ERR_MASK))
+				s->flags |= SF_ERR_INTERNAL;
+			return 0;
+		}
 
 		/* we may request to parse a request body */
 		if (be->options & PR_O_WREQ_BODY)