diff --git a/dev/flake-module.nix b/dev/flake-module.nix
index 5ac60b94..64976b7b 100644
--- a/dev/flake-module.nix
+++ b/dev/flake-module.nix
@@ -48,4 +48,9 @@
       };
     });
   };
+
+  # Make some non-standard flake outputs available for exploration in the REPL
+  # This isn't part of a proper flake anyway (see dev/flake.nix) so we may as
+  # well leave this on.
+  debug = true;
 }
diff --git a/lib.nix b/lib.nix
index 64c35c12..89098cb1 100644
--- a/lib.nix
+++ b/lib.nix
@@ -37,8 +37,11 @@ let
     attrs@{ staticModules ? [ ] }: mkOptionType {
       name = "deferredModule";
       description = "module";
+      descriptionClass = "noun";
       check = x: isAttrs x || isFunction x || path.check x;
-      merge = loc: defs: staticModules ++ map (def: lib.setDefaultModuleLocation "${def.file}, via option ${showOption loc}" def.value) defs;
+      merge = loc: defs: {
+        imports = staticModules ++ map (def: lib.setDefaultModuleLocation "${def.file}, via option ${showOption loc}" def.value) defs;
+      };
       inherit (submoduleWith { modules = staticModules; })
         getSubOptions
         getSubModules;
diff --git a/modules/apps.nix b/modules/apps.nix
index 0fd5d7c6..e3952387 100644
--- a/modules/apps.nix
+++ b/modules/apps.nix
@@ -15,7 +15,9 @@ let
   };
 
   getExe = x:
-    "${lib.getBin x}/bin/${x.meta.mainProgram or (throw ''Package ${x.name or ""} does not have meta.mainProgram set, so I don't know how to find the main executable. You can set meta.mainProgram, or pass the full path to executable, e.g. program = "''${pkg}/bin/foo"'')}";
+    "${lib.getBin x}/bin/${x.meta.mainProgram or (throw
+      ''Package ${x.name or ""} does not have meta.mainProgram set, so I don't know how to find the main executable. You can set `meta.mainProgram`, or pass the full path to executable, e.g. program = "''${pkg}/bin/foo"''
+    )}";
 
   appType = lib.types.submodule {
     options = {
diff --git a/modules/moduleWithSystem.nix b/modules/moduleWithSystem.nix
index e5c70083..e4d397f5 100644
--- a/modules/moduleWithSystem.nix
+++ b/modules/moduleWithSystem.nix
@@ -9,22 +9,23 @@
         let
           system =
             config._module.args.system or
-              config._module.args.pkgs.stdenv.hostPlatform.system or
-                (throw "moduleWithSystem: Could not determine the configuration's system parameter for this module system application.");
+              config._module.args.pkgs.stdenv.hostPlatform.system or (throw
+                "moduleWithSystem: Could not determine the `system` parameter for this module set evaluation."
+              );
 
-          allArgs = withSystem system (args: args);
+          allPerSystemArgs = withSystem system (args: args);
 
-          lazyArgsPerParameter = f: builtins.mapAttrs
-            (k: v: allArgs.${k} or (throw "moduleWithSystem: module argument `${k}` does not exist."))
+          lazyPerSystemArgsPerParameter = f: builtins.mapAttrs
+            (k: v: allPerSystemArgs.${k} or (throw "moduleWithSystem: per-system argument `${k}` does not exist."))
             (builtins.functionArgs f);
 
           # Use reflection to make the call lazy in the argument.
           # Restricts args to the ones declared.
-          callLazily = f: a: f (lazyArgsPerParameter f);
+          callLazily = f: a: f (lazyPerSystemArgsPerParameter f);
         in
         {
           imports = [
-            (callLazily module allArgs)
+            (callLazily module allPerSystemArgs)
           ];
         };
     };
diff --git a/modules/nixpkgs.nix b/modules/nixpkgs.nix
index 44df9158..730af495 100644
--- a/modules/nixpkgs.nix
+++ b/modules/nixpkgs.nix
@@ -13,11 +13,13 @@
 #
 {
   config = {
-    perSystem = { inputs', lib, ... }: {
+    perSystem = { inputs', lib, options, ... }: {
       config = {
         _module.args.pkgs = lib.mkOptionDefault (
           builtins.seq
-            (inputs'.nixpkgs or (throw "flake-parts: The flake does not have a `nixpkgs` input. Please add it, or set `perSystem._module.args.pkgs` yourself."))
+            inputs'.nixpkgs or (throw
+              "flake-parts: The flake does not have a `nixpkgs` input. Please add it, or set `${options._module.args}.pkgs` yourself."
+            )
             inputs'.nixpkgs.legacyPackages
         );
       };
diff --git a/modules/perSystem.nix b/modules/perSystem.nix
index 499858a2..4b20e55b 100644
--- a/modules/perSystem.nix
+++ b/modules/perSystem.nix
@@ -119,9 +119,9 @@ in
           _module.args.moduleWithSystem = throwAliasError "moduleWithSystem";
         };
       });
-      apply = modules: system:
+      apply = module: system:
         (lib.evalModules {
-          inherit modules;
+          modules = [ module ];
           prefix = [ "perSystem" system ];
           specialArgs = {
             inherit system;