diff --git a/lib/modules.nix b/lib/modules.nix
index 7ad72235994..076ffdde246 100644
--- a/lib/modules.nix
+++ b/lib/modules.nix
@@ -6,16 +6,14 @@ rec {
   /* Evaluate a set of modules.  The result is a set of two
      attributes: ‘options’: the nested set of all option declarations,
      and ‘config’: the nested set of all option values. */
-  evalModules = evalModules' [];
-
-  evalModules' = prefix: modules: args:
+  evalModules = { modules, prefix ? [], args ? {}, check ? true }:
     let
       args' = args // result;
       closed = closeModules modules args';
       # Note: the list of modules is reversed to maintain backward
       # compatibility with the old module system.  Not sure if this is
       # the most sensible policy.
-      options = mergeModules prefix (reverseList closed);
+      options = mergeModules check prefix (reverseList closed);
       config = yieldConfig options;
       yieldConfig = mapAttrs (n: v: if isOption v then v.value else yieldConfig v);
       result = { inherit options config; };
@@ -52,7 +50,7 @@ rec {
       { inherit file key;
         imports = m.require or [];
         options = {};
-        config = m;
+        config = removeAttrs m ["require"];
       };
 
   applyIfFunction = f: arg: if builtins.isFunction f then f arg else f;
@@ -62,13 +60,19 @@ rec {
      At the same time, for each option declaration, it will merge the
      corresponding option definitions in all machines, returning them
      in the ‘value’ attribute of each option. */
-  mergeModules = prefix: modules:
-    mergeModules' prefix modules
+  mergeModules = check: prefix: modules:
+    mergeModules' check prefix modules
       (concatMap (m: map (config: { inherit (m) file; inherit config; }) (pushDownProperties m.config)) modules);
 
-  mergeModules' = prefix: options: configs:
-    let names = concatMap (m: attrNames m.options) options;
-    in listToAttrs (map (name: {
+  mergeModules' = check: prefix: options: configs:
+    let
+      declaredNames = concatMap (m: attrNames m.options) options;
+      declaredNames' = listToAttrs (map (n: { name = n; value = 1; }) declaredNames);
+      checkDefinedNames = res: fold (m: res: fold (n: res:
+          if hasAttr n declaredNames' then res else
+            throw "The option `${showOption (prefix ++ [n])}' defined in `${m.file}' does not exist."
+        ) res (attrNames m.config)) res configs;
+    in (if check then checkDefinedNames else id) (listToAttrs (map (name: {
       # We're descending into attribute ‘name’.
       inherit name;
       value =
@@ -105,8 +109,8 @@ rec {
             in
               throw "The option `${showOption loc}' in `${firstOption.file}' is a prefix of options in `${firstNonOption.file}'."
           else
-            mergeModules' loc decls defns;
-    }) names);
+            mergeModules' check loc decls defns;
+    }) declaredNames));
 
   /* Merge multiple option declarations into a single declaration.  In
      general, there should be only one declaration of each option.
diff --git a/lib/types.nix b/lib/types.nix
index f459cda73cb..07a6cc69fdc 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -182,11 +182,11 @@ rec {
           let
             coerce = def: if builtins.isFunction def then def else { config = def; };
             modules = opts' ++ map coerce defs;
-          in (evalModules' args.prefix modules args).config;
-        getSubOptions = prefix: (evalModules' prefix opts'
-          # FIXME: hack to get shit to evaluate.
-          { name = ""; }
-        ).options;
+          in (evalModules { inherit modules args; prefix = args.prefix; }).config;
+        getSubOptions = prefix: (evalModules
+          { modules = opts'; inherit prefix;
+            # FIXME: hack to get shit to evaluate.
+            args = { name = ""; }; }).options;
       };
 
     # Obsolete alternative to configOf.  It takes its option
diff --git a/nixos/lib/eval-config.nix b/nixos/lib/eval-config.nix
index 119bba78ff1..5d487b91afb 100644
--- a/nixos/lib/eval-config.nix
+++ b/nixos/lib/eval-config.nix
@@ -7,19 +7,21 @@
 , baseModules ? import ../modules/module-list.nix
 , extraArgs ? {}
 , modules
+, check ? true
 }:
 
 let extraArgs_ = extraArgs; pkgs_ = pkgs; system_ = system; in
 
 rec {
 
-  # These are the NixOS modules that constitute the system configuration.
-  configComponents = modules ++ baseModules;
-
   # Merge the option definitions in all modules, forming the full
-  # system configuration.  It's not checked for undeclared options.
+  # system configuration.
   systemModule =
-    pkgs.lib.evalModules configComponents extraArgs;
+    pkgs.lib.evalModules {
+      modules = modules ++ baseModules;
+      args = extraArgs;
+      inherit check;
+    };
 
   config = systemModule.config;
 
@@ -54,6 +56,7 @@ rec {
           # define nixpkgs.config, so it's pointless to evaluate them.
           baseModules = [ ../modules/misc/nixpkgs.nix ];
           pkgs = import ./nixpkgs.nix { system = system_; config = {}; };
+          check = false;
         }).config.nixpkgs;
       in
       {
diff --git a/nixos/modules/services/misc/nixos-manual.nix b/nixos/modules/services/misc/nixos-manual.nix
index 66d46d9114a..f67afb747e9 100644
--- a/nixos/modules/services/misc/nixos-manual.nix
+++ b/nixos/modules/services/misc/nixos-manual.nix
@@ -16,13 +16,15 @@ let
       system.nixosRevision = config.system.nixosRevision;
     };
 
+  eval = evalModules {
+    modules = [ versionModule ] ++ baseModules;
+    args = (removeAttrs extraArgs ["config" "options"]) // { modules = [ ]; };
+  };
+
   manual = import ../../../doc/manual {
     inherit pkgs;
     revision = config.system.nixosRevision;
-    options = (evalModules ([ versionModule ] ++ baseModules)
-      (removeAttrs extraArgs ["config" "options"]) // {
-        modules = [ ];
-      }).options;
+    options = eval.options;
   };
 
   entry = "${manual.manual}/share/doc/nixos/manual.html";