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;
       };
 
   };