diff --git a/nixos/modules/services/monitoring/prometheus/alertmanager.nix b/nixos/modules/services/monitoring/prometheus/alertmanager.nix
index cf761edad926..8a47c9f1e7d8 100644
--- a/nixos/modules/services/monitoring/prometheus/alertmanager.nix
+++ b/nixos/modules/services/monitoring/prometheus/alertmanager.nix
@@ -111,11 +111,11 @@ in {
       after    = [ "network.target" ];
       script = ''
         ${pkgs.prometheus-alertmanager.bin}/bin/alertmanager \
-        -config.file ${alertmanagerYml} \
-        -web.listen-address ${cfg.listenAddress}:${toString cfg.port} \
-        -log.level ${cfg.logLevel} \
-        ${optionalString (cfg.webExternalUrl != null) ''-web.external-url ${cfg.webExternalUrl} \''}
-        ${optionalString (cfg.logFormat != null) "-log.format ${cfg.logFormat}"}
+        --config.file ${alertmanagerYml} \
+        --web.listen-address ${cfg.listenAddress}:${toString cfg.port} \
+        --log.level ${cfg.logLevel} \
+        ${optionalString (cfg.webExternalUrl != null) ''--web.external-url ${cfg.webExternalUrl} \''}
+        ${optionalString (cfg.logFormat != null) "--log.format ${cfg.logFormat}"}
       '';
 
       serviceConfig = {