diff --git a/src/prometheus/tiny_httpd_prometheus.ml b/src/prometheus/tiny_httpd_prometheus.ml
index dcc19a8b..b1731d5b 100644
--- a/src/prometheus/tiny_httpd_prometheus.ml
+++ b/src/prometheus/tiny_httpd_prometheus.ml
@@ -200,7 +200,7 @@ let http_middleware (reg : Registry.t) : H.Middleware.t =
          let elapsed_s = elapsed_us /. 1e6 in
          Histogram.add h_latency elapsed_s;
 
-         if code < 200 || code >= 300 then Counter.incr c_err;
+         if code < 200 || code >= 400 then Counter.incr c_err;
          resp response)
 
 let add_route_to_server (server : H.t) (reg : registry) : unit =