diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index c29ee97d8..75f463ba7 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -3206,6 +3206,13 @@ ngx_http_upstream_upgrade(ngx_http_request_t *r, ngx_http_upstream_t *u)
 
     /* TODO: prevent upgrade if not requested or not possible */
 
+    if (r != r->main) {
+        ngx_log_error(NGX_LOG_ERR, c->log, 0,
+                      "connection upgrade in subrequest");
+        ngx_http_upstream_finalize_request(r, u, NGX_ERROR);
+        return;
+    }
+
     r->keepalive = 0;
     c->log->action = "proxying upgraded connection";