diff --git a/nixos/modules/services/monitoring/prometheus/default.nix b/nixos/modules/services/monitoring/prometheus/default.nix
index 6ff662b53cb8..712f55672818 100644
--- a/nixos/modules/services/monitoring/prometheus/default.nix
+++ b/nixos/modules/services/monitoring/prometheus/default.nix
@@ -57,9 +57,9 @@ let
     "-web.listen-address=${cfg.listenAddress}"
     "-alertmanager.notification-queue-capacity=${toString cfg.alertmanagerNotificationQueueCapacity}"
     "-alertmanager.timeout=${toString cfg.alertmanagerTimeout}s"
-    (optionalString (cfg.alertmanagerURL != []) "-alertmanager.url=${concatStringsSep "," cfg.alertmanagerURL}")
-    (optionalString (cfg.webExternalUrl != null) "-web.external-url=${cfg.webExternalUrl}")
-  ];
+  ] ++
+    (optional (cfg.alertmanagerURL != []) "-alertmanager.url=${concatStringsSep "," cfg.alertmanagerURL}") ++
+    (optional (cfg.webExternalUrl != null) "-web.external-url=${cfg.webExternalUrl}");
 
   # This becomes the main config file for Prometheus 2
   promConfig2 = {
@@ -91,8 +91,8 @@ let
     "--web.listen-address=${cfg2.listenAddress}"
     "--alertmanager.notification-queue-capacity=${toString cfg2.alertmanagerNotificationQueueCapacity}"
     "--alertmanager.timeout=${toString cfg2.alertmanagerTimeout}s"
-    (optionalString (cfg2.webExternalUrl != null) "--web.external-url=${cfg2.webExternalUrl}")
-  ];
+  ] ++
+    (optional (cfg2.webExternalUrl != null) "--web.external-url=${cfg2.webExternalUrl}");
 
   promTypes.globalConfig = types.submodule {
     options = {