diff --git a/nixos/modules/services/misc/nix-optimise.nix b/nixos/modules/services/misc/nix-optimise.nix
index fea322a68f8e..87ce05c5a11b 100644
--- a/nixos/modules/services/misc/nix-optimise.nix
+++ b/nixos/modules/services/misc/nix-optimise.nix
@@ -21,7 +21,7 @@ in
       };
 
       dates = mkOption {
-        default = "03:45";
+        default = ["03:45"];
         type = types.listOf types.str;
         description = ''
           Specification (in the format described by