diff --git a/buildbot_nix/__init__.py b/buildbot_nix/__init__.py
index a06c29e..5016e46 100644
--- a/buildbot_nix/__init__.py
+++ b/buildbot_nix/__init__.py
@@ -750,6 +750,7 @@ class NixConfigurator(ConfiguratorBase):
         nix_eval_max_memory_size: int,
         nix_workers_secret_name: str = "buildbot-nix-workers",
         outputs_path: str | None = None,
+        prometheus_exporter_port: int | None = None,
     ) -> None:
         super().__init__()
         self.nix_workers_secret_name = nix_workers_secret_name
@@ -763,6 +764,7 @@ class NixConfigurator(ConfiguratorBase):
             self.outputs_path = None
         else:
             self.outputs_path = Path(outputs_path)
+        self.prometheus_exporter_port = prometheus_exporter_port
 
     def configure(self, config: dict[str, Any]) -> None:
         projects = load_projects(self.github.token(), self.github.project_cache_file)
@@ -839,13 +841,20 @@ class NixConfigurator(ConfiguratorBase):
                 context=Interpolate("buildbot/%(prop:status_name)s"),
             )
         )
+        if self.prometheus_exporter_port:
+            config["services"].append(
+                reporters.Prometheus(port=self.prometheus_exporter_port)
+            )
+
         systemd_secrets = secrets.SecretInAFile(
             dirname=os.environ["CREDENTIALS_DIRECTORY"]
         )
         config["secretsProviders"].append(systemd_secrets)
-        config["www"]["change_hook_dialects"] = config["www"].get(
-            "change_hook_dialects", {}
-        )
+
+        config["www"].setdefault("plugins", {})
+        config["www"]["plugins"].update(dict(base_react={}))
+
+        config["www"].setdefault("change_hook_dialects", {})
         config["www"]["change_hook_dialects"]["github"] = {
             "secret": webhook_secret,
             "strict": True,
@@ -853,8 +862,8 @@ class NixConfigurator(ConfiguratorBase):
             "github_property_whitelist": "*",
         }
 
-        if not config["www"].get("auth"):
-            config["www"]["avatar_methods"] = config["www"].get("avatar_methods", [])
+        if "auth" not in config["www"]:
+            config["www"].setdefault("avatar_methods", [])
             config["www"]["avatar_methods"].append(
                 util.AvatarGitHub(token=self.github.token())
             )
diff --git a/nix/master.nix b/nix/master.nix
index 20feac5..369c7ca 100644
--- a/nix/master.nix
+++ b/nix/master.nix
@@ -126,15 +126,6 @@ in
         from datetime import timedelta
         from buildbot_nix import GithubConfig, NixConfigurator
       '';
-      extraConfig = ''
-        c["www"]["plugins"] = c["www"].get("plugins", {})
-        c["www"]["plugins"].update(
-            dict(base_react={})
-        )
-        ${lib.optionalString (cfg.prometheusExporterPort != null) ''
-          c['services'].append(reporters.Prometheus(port=${builtins.toString cfg.prometheusExporterPort}))
-        ''}
-      '';
       configurators = [
         ''
           util.JanitorConfigurator(logHorizon=timedelta(weeks=4), hour=12, dayOfWeek=6)
@@ -152,6 +143,7 @@ in
               nix_eval_worker_count=${builtins.toJSON cfg.evalWorkerCount},
               nix_supported_systems=${builtins.toJSON cfg.buildSystems},
               outputs_path=${if cfg.outputsPath == null then "None" else builtins.toJSON cfg.outputsPath},
+              prometheus_exporter_port=${if cfg.prometheusExporterPort == null then "None" else builtins.toJSON cfg.prometheusExporterPort},
           )
         ''
       ];