diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index ba21c0135..577c2b563 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -1591,7 +1591,7 @@ ngx_http_upstream_process_header(ngx_http_request_t *r, ngx_http_upstream_t *u)
 
         if (rc == NGX_AGAIN) {
 
-            if (u->buffer.pos == u->buffer.end) {
+            if (u->buffer.last == u->buffer.end) {
                 ngx_log_error(NGX_LOG_ERR, c->log, 0,
                               "upstream sent too big header");