diff --git a/modules/services/nix-gc.nix b/modules/services/nix-gc.nix
index d95d6e94c..a49220990 100644
--- a/modules/services/nix-gc.nix
+++ b/modules/services/nix-gc.nix
@@ -66,7 +66,7 @@ let
         ];
       };
     in
-    freq.${frequency};
+    freq.${frequency} or null;
 
   nixPackage =
     if config.nix.enable && config.nix.package != null then config.nix.package else pkgs.nix;
@@ -95,7 +95,7 @@ in
 
           On Linux this is a string as defined by {manpage}`systemd.time(7)`.
 
-          On Darwin it must be one of: ${toString darwinIntervals}, which are
+          On Darwin it must be one of: ${lib.concatStringsSep ", " darwinIntervals}, which are
           implemented as defined in the manual page above.
         '';
       };
@@ -172,7 +172,7 @@ in
         assertions = [
           {
             assertion = lib.elem cfg.frequency darwinIntervals;
-            message = "On Darwin nix.gc.frequency must be one of: ${toString darwinIntervals}.";
+            message = "On Darwin nix.gc.frequency must be one of: ${lib.concatStringsSep ", " darwinIntervals}.";
           }
         ];
 
diff --git a/tests/modules/services/nix-gc-darwin/darwin-nix-gc-interval-assertion.nix b/tests/modules/services/nix-gc-darwin/darwin-nix-gc-interval-assertion.nix
new file mode 100644
index 000000000..6dbf70e83
--- /dev/null
+++ b/tests/modules/services/nix-gc-darwin/darwin-nix-gc-interval-assertion.nix
@@ -0,0 +1,10 @@
+{
+  nix.gc = {
+    automatic = true;
+    frequency = "00:02:03";
+  };
+
+  test.asserts.assertions.expected = [
+    "On Darwin nix.gc.frequency must be one of: hourly, daily, weekly, monthly, semiannually, annually."
+  ];
+}
diff --git a/tests/modules/services/nix-gc-darwin/default.nix b/tests/modules/services/nix-gc-darwin/default.nix
index f2fc20aa7..251e0d23a 100644
--- a/tests/modules/services/nix-gc-darwin/default.nix
+++ b/tests/modules/services/nix-gc-darwin/default.nix
@@ -1 +1,4 @@
-{ nix-gc = ./basic.nix; }
+{
+  nix-gc = ./basic.nix;
+  darwin-nix-gc-interval-assertion = ./darwin-nix-gc-interval-assertion.nix;
+}