diff --git a/lib/modules.nix b/lib/modules.nix
index e9fc1007fc20..79d54e4a5387 100644
--- a/lib/modules.nix
+++ b/lib/modules.nix
@@ -151,8 +151,7 @@ rec {
           };
 
           _module.freeformType = mkOption {
-            # Disallow merging for now, but could be implemented nicely with a `types.optionType`
-            type = types.nullOr (types.uniq types.attrs);
+            type = types.nullOr types.optionType;
             internal = true;
             default = null;
             description = ''
diff --git a/lib/tests/modules.sh b/lib/tests/modules.sh
index d11f32e59964..e4bb7ad21900 100755
--- a/lib/tests/modules.sh
+++ b/lib/tests/modules.sh
@@ -240,6 +240,11 @@ checkConfigOutput '^"24"$' config.foo ./freeform-attrsOf.nix ./freeform-str-dep-
 checkConfigError 'infinite recursion encountered' config.foo ./freeform-attrsOf.nix ./freeform-unstr-dep-str.nix
 checkConfigError 'The option .* is used but not defined' config.foo ./freeform-lazyAttrsOf.nix ./freeform-unstr-dep-str.nix
 checkConfigOutput '^"24"$' config.foo ./freeform-lazyAttrsOf.nix ./freeform-unstr-dep-str.nix ./define-value-string.nix
+# submodules in freeformTypes should have their locations annotated
+checkConfigOutput '/freeform-submodules.nix"$' config.fooDeclarations.0 ./freeform-submodules.nix
+# freeformTypes can get merged using `types.type`, including submodules
+checkConfigOutput '^10$' config.free.xxx.foo ./freeform-submodules.nix
+checkConfigOutput '^10$' config.free.yyy.bar ./freeform-submodules.nix
 
 ## types.anything
 # Check that attribute sets are merged recursively
diff --git a/lib/tests/modules/freeform-submodules.nix b/lib/tests/modules/freeform-submodules.nix
new file mode 100644
index 000000000000..3910435a7b5c
--- /dev/null
+++ b/lib/tests/modules/freeform-submodules.nix
@@ -0,0 +1,22 @@
+{ lib, options, ... }: with lib.types; {
+
+  options.fooDeclarations = lib.mkOption {
+    default = (options.free.type.getSubOptions [])._freeformOptions.foo.declarations;
+  };
+
+  options.free = lib.mkOption {
+    type = submodule {
+      config._module.freeformType = lib.mkMerge [
+        (attrsOf (submodule {
+          options.foo = lib.mkOption {};
+        }))
+        (attrsOf (submodule {
+          options.bar = lib.mkOption {};
+        }))
+      ];
+    };
+  };
+
+  config.free.xxx.foo = 10;
+  config.free.yyy.bar = 10;
+}