diff --git a/nixos/modules/installer/tools/auto-upgrade.nix b/nixos/modules/installer/tools/auto-upgrade.nix
index b21b80c666a..dfb43d1a1db 100644
--- a/nixos/modules/installer/tools/auto-upgrade.nix
+++ b/nixos/modules/installer/tools/auto-upgrade.nix
@@ -84,7 +84,7 @@ let cfg = config.system.autoUpgrade; in
         ${config.system.build.nixos-rebuild}/bin/nixos-rebuild switch ${toString cfg.flags}
       '';
 
-      startAt = optionalString cfg.enable cfg.dates;
+      startAt = optional cfg.enable cfg.dates;
     };
 
   };
diff --git a/nixos/modules/services/misc/nix-gc.nix b/nixos/modules/services/misc/nix-gc.nix
index 5c13da6e83d..304168c65b0 100644
--- a/nixos/modules/services/misc/nix-gc.nix
+++ b/nixos/modules/services/misc/nix-gc.nix
@@ -53,7 +53,7 @@ in
     systemd.services.nix-gc =
       { description = "Nix Garbage Collector";
         script = "exec ${config.nix.package.out}/bin/nix-collect-garbage ${cfg.options}";
-        startAt = optionalString cfg.automatic cfg.dates;
+        startAt = optional cfg.automatic cfg.dates;
       };
 
   };