diff --git a/docs/options.nix b/docs/options.nix
index 3d3c484..b4c2a8f 100644
--- a/docs/options.nix
+++ b/docs/options.nix
@@ -8,8 +8,11 @@ let
     options = eval.options;
   };
 
-in pkgs.writeText "agent-options" ''
+in (pkgs.writeText "agent-options" ''
   = Arion options
 
   ${options.optionsAsciiDoc}
-''
+'').overrideAttrs (o: {
+  # Work around https://github.com/hercules-ci/hercules-ci-agent/issues/168
+  allowSubstitutes = true;
+})
diff --git a/nix/ci.nix b/nix/ci.nix
index ed61e2e..0d37467 100644
--- a/nix/ci.nix
+++ b/nix/ci.nix
@@ -24,15 +24,16 @@ dimension "Nixpkgs version" {
 
       dimension "System" {
         "x86_64-linux" = { isReferenceTarget = isReferenceNixpkgs; };
-        # TODO: darwin
-        # "x86_64-darwin" = { enableNixOSTests = false; };
+        "x86_64-darwin" = { enableNixOSTests = false; };
       } (
-        system: { isReferenceTarget ? false }:
+        system: { isReferenceTarget ? false, enableNixOSTests ? true }:
           let
             pkgs = import ./. { inherit system; nixpkgsSrc = sources.${nixpkgsSource}; };
           in
           {
-            inherit (pkgs) arion tests;
+            inherit (pkgs) arion;
+          } // lib.optionalAttrs enableNixOSTests {
+            inherit (pkgs) tests;
           } // lib.optionalAttrs enableDoc {
             inherit (pkgs) doc doc-options doc-options-check;
           } // lib.optionalAttrs isReferenceTarget {
diff --git a/src/haskell/test/Arion/NixSpec.hs b/src/haskell/test/Arion/NixSpec.hs
index f5c3887..e20bebf 100644
--- a/src/haskell/test/Arion/NixSpec.hs
+++ b/src/haskell/test/Arion/NixSpec.hs
@@ -18,7 +18,7 @@ spec = describe "evaluateComposition" $ it "matches an example" $ do
     { evalUid      = 123
     , evalModules  = NEL.fromList
                        ["src/haskell/testdata/Arion/NixSpec/arion-compose.nix"]
-    , evalPkgs     = "import <nixpkgs> {}"
+    , evalPkgs     = "import <nixpkgs> { system = \"x86_64-linux\"; }"
     , evalWorkDir  = Nothing
     , evalMode     = ReadOnly
     , evalUserArgs = ["--show-trace"]