diff --git a/modules/lib/darwin.nix b/modules/lib/darwin.nix
index 070cf2153..8c05a3d97 100644
--- a/modules/lib/darwin.nix
+++ b/modules/lib/darwin.nix
@@ -63,12 +63,12 @@ let
   intervalsString = lib.concatStringsSep ", " intervals;
 
   assertInterval = option: interval: pkgs: {
-    assertion = (!pkgs.stdenv.isDarwin) || (lib.elem interval intervals);
-    message = "On Darwin ${option} must be one of: ${intervalsString}.";
+    assertion = pkgs.stdenv.isDarwin -> lib.elem interval intervals;
+    message = "On Darwin, ${option} must be one of: ${intervalsString}.";
   };
 
   intervalDocumentation = ''
-    On Darwin it must be one of: ${intervalsString}, which are implemented as defined in {manpage}`systemd.time(7)`.
+    On Darwin, it must be one of: ${intervalsString}, which are implemented as defined in {manpage}`systemd.time(7)`.
   '';
 in
 {
diff --git a/tests/modules/services/borgmatic/darwin/frequency-assertion.nix b/tests/modules/services/borgmatic/darwin/frequency-assertion.nix
index f8ebb0c22..64b1a91d0 100644
--- a/tests/modules/services/borgmatic/darwin/frequency-assertion.nix
+++ b/tests/modules/services/borgmatic/darwin/frequency-assertion.nix
@@ -5,6 +5,6 @@
   };
 
   test.asserts.assertions.expected = [
-    "On Darwin services.borgmatic.frequency must be one of: hourly, daily, weekly, monthly, semiannually, annually."
+    "On Darwin, services.borgmatic.frequency must be one of: hourly, daily, weekly, monthly, semiannually, annually."
   ];
 }
diff --git a/tests/modules/services/home-manager-auto-expire/darwin/frequency-assertion.nix b/tests/modules/services/home-manager-auto-expire/darwin/frequency-assertion.nix
index 568d20439..90212448c 100644
--- a/tests/modules/services/home-manager-auto-expire/darwin/frequency-assertion.nix
+++ b/tests/modules/services/home-manager-auto-expire/darwin/frequency-assertion.nix
@@ -5,6 +5,6 @@
   };
 
   test.asserts.assertions.expected = [
-    "On Darwin services.home-manager.autoExpire.frequency must be one of: hourly, daily, weekly, monthly, semiannually, annually."
+    "On Darwin, services.home-manager.autoExpire.frequency must be one of: hourly, daily, weekly, monthly, semiannually, annually."
   ];
 }
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
index f3bedb68d..e1e22d0a7 100644
--- 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
@@ -5,6 +5,6 @@
   };
 
   test.asserts.assertions.expected = [
-    "On Darwin nix.gc.dates.* must be one of: hourly, daily, weekly, monthly, semiannually, annually."
+    "On Darwin, nix.gc.dates.* must be one of: hourly, daily, weekly, monthly, semiannually, annually."
   ];
 }