diff --git a/lib/options.nix b/lib/options.nix
index 5d52f065af084..c7970d38e733f 100644
--- a/lib/options.nix
+++ b/lib/options.nix
@@ -185,7 +185,13 @@ rec {
             then true
             else opt.visible or true;
           readOnly = opt.readOnly or false;
-          type = opt.type.description or null;
+          type =
+            if opt.type.name == "unspecified"
+            then lib.warn
+              ("Option `${showOption loc}' defined in ${showFiles opt.declarations} has no type.\n"
+              +"       Please specify a valid types from <nixpkgs/lib/types.nix> or use types.raw if there is no appropriate type.")
+              null
+            else opt.type.description or null;
         }
         // optionalAttrs (opt ? example) { example = scrubOptionValue opt.example; }
         // optionalAttrs (opt ? default) { default = scrubOptionValue opt.default; }
diff --git a/nixos/doc/manual/development/option-declarations.section.md b/nixos/doc/manual/development/option-declarations.section.md
index be56529992ab1..d2fde88c4522d 100644
--- a/nixos/doc/manual/development/option-declarations.section.md
+++ b/nixos/doc/manual/development/option-declarations.section.md
@@ -27,9 +27,8 @@ The function `mkOption` accepts the following arguments.
 
 `type`
 
-:   The type of the option (see [](#sec-option-types)). It may be
-    omitted, but that's not advisable since it may lead to errors that
-    are hard to diagnose.
+:   The type of the option (see [](#sec-option-types)). This
+    argument is mandatory.
 
 `default`
 
diff --git a/nixos/doc/manual/from_md/development/option-declarations.section.xml b/nixos/doc/manual/from_md/development/option-declarations.section.xml
index 2845e37659b1b..7954dee1d9c00 100644
--- a/nixos/doc/manual/from_md/development/option-declarations.section.xml
+++ b/nixos/doc/manual/from_md/development/option-declarations.section.xml
@@ -38,9 +38,8 @@ options = {
       <listitem>
         <para>
           The type of the option (see
-          <xref linkend="sec-option-types" />). It may be omitted, but
-          that’s not advisable since it may lead to errors that are hard
-          to diagnose.
+          <xref linkend="sec-option-types" />). This argument is
+          mandatory.
         </para>
       </listitem>
     </varlistentry>
diff --git a/nixos/modules/services/networking/nsd.nix b/nixos/modules/services/networking/nsd.nix
index cf6c9661dc1b0..ed2ab88835ff0 100644
--- a/nixos/modules/services/networking/nsd.nix
+++ b/nixos/modules/services/networking/nsd.nix
@@ -247,6 +247,7 @@ let
 
       children = mkOption {
         default = {};
+        type = types.attrsOf types.unspecified;
         description = ''
           Children zones inherit all options of their parents. Attributes
           defined in a child will overwrite the ones of its parent. Only
diff --git a/nixos/modules/services/networking/unbound.nix b/nixos/modules/services/networking/unbound.nix
index f6e9634909241..87873c8c1e83b 100644
--- a/nixos/modules/services/networking/unbound.nix
+++ b/nixos/modules/services/networking/unbound.nix
@@ -62,6 +62,7 @@ in {
       };
 
       stateDir = mkOption {
+        type = types.path;
         default = "/var/lib/unbound";
         description = "Directory holding all state for unbound to run.";
       };
diff --git a/nixos/modules/services/networking/vsftpd.nix b/nixos/modules/services/networking/vsftpd.nix
index 710c2d9ca17b6..d205302051e14 100644
--- a/nixos/modules/services/networking/vsftpd.nix
+++ b/nixos/modules/services/networking/vsftpd.nix
@@ -153,6 +153,7 @@ in
 
       userlist = mkOption {
         default = [];
+        type = types.listOf types.str;
         description = "See <option>userlistFile</option>.";
       };
 
diff --git a/nixos/modules/services/x11/display-managers/default.nix b/nixos/modules/services/x11/display-managers/default.nix
index bdc46faa7fd0c..b3820a90ca42d 100644
--- a/nixos/modules/services/x11/display-managers/default.nix
+++ b/nixos/modules/services/x11/display-managers/default.nix
@@ -217,6 +217,24 @@ in
 
       session = mkOption {
         default = [];
+        type = with types; listOf (submodule ({ ... }: {
+          options = {
+            manage = mkOption {
+              description = "Whether this is a desktop or a window manager";
+              type = enum [ "desktop" "window" ];
+            };
+
+            name = mkOption {
+              description = "Name of this session";
+              type = str;
+            };
+
+            start = mkOption {
+              description = "Commands to run to start this session";
+              type = lines;
+            };
+          };
+        }));
         example = literalExpression
           ''
             [ { manage = "desktop";
diff --git a/nixos/modules/system/boot/kernel.nix b/nixos/modules/system/boot/kernel.nix
index d147155d796c1..dde70dc5ab6a5 100644
--- a/nixos/modules/system/boot/kernel.nix
+++ b/nixos/modules/system/boot/kernel.nix
@@ -36,7 +36,7 @@ in
 
     boot.kernelPackages = mkOption {
       default = pkgs.linuxPackages;
-      type = types.unspecified // { merge = mergeEqualOption; };
+      type = types.anything // { merge = mergeEqualOption; };
       apply = kernelPackages: kernelPackages.extend (self: super: {
         kernel = super.kernel.override (originalArgs: {
           inherit randstructSeed;
diff --git a/nixos/modules/system/boot/stage-1.nix b/nixos/modules/system/boot/stage-1.nix
index 409424a5b0f65..14eaf7596ec35 100644
--- a/nixos/modules/system/boot/stage-1.nix
+++ b/nixos/modules/system/boot/stage-1.nix
@@ -575,7 +575,7 @@ in
         else "gzip"
       );
       defaultText = literalDocBook "<literal>zstd</literal> if the kernel supports it (5.9+), <literal>gzip</literal> if not";
-      type = types.unspecified; # We don't have a function type...
+      type = types.anything; # We don't have a function type...
       description = ''
         The compressor to use on the initrd image. May be any of: