diff --git a/app/Http/Controllers/WikiSettingController.php b/app/Http/Controllers/WikiSettingController.php
index a96692ec..8e8b4f99 100644
--- a/app/Http/Controllers/WikiSettingController.php
+++ b/app/Http/Controllers/WikiSettingController.php
@@ -13,10 +13,14 @@ class WikiSettingController extends Controller {
     /**
      * @return (SettingWikibaseManifestEquivEntities|string)[][]
      */
-    private function getSettingValidations(): array {
+    private function getSettingValidations(bool $allowModernSkin = false): array {
         // FIXME: this list is evil and should be kept in sync with the model in Wiki.php?! (mostly)
+        $defaultSkinValidation = $allowModernSkin
+            ? 'in:vector,modern,timeless'
+            : 'in:vector,timeless';
+
         return [
-            'wgDefaultSkin' => ['required', 'string', 'in:vector,modern,timeless'],
+            'wgDefaultSkin' => ['required', 'string', $defaultSkinValidation],
             'wwExtEnableConfirmAccount' => ['required', 'boolean'],
             'wwExtEnableWikibaseLexeme' => ['required', 'boolean'],
             'wwWikibaseStringLengthString' => ['required', 'integer', 'between:400,2500'],
@@ -40,10 +44,17 @@ public function update($setting, Request $request) {
             'setting' => 'required|string|in:' . implode(',', array_keys($settingValidations)),
         ]);
         $settingName = $request->input('setting');
+        $wiki = $request->attributes->get('wiki');
+
+        if ($settingName === 'wgDefaultSkin' && $request->input('value') === 'modern') {
+            $hasModernDefault = $wiki->settings()
+                ->where('name', 'wgDefaultSkin')
+                ->value('value') === 'modern';
+            $settingValidations = $this->getSettingValidations($hasModernDefault);
+        }
 
         $request->validate(['value' => $settingValidations[$settingName]]);
         $value = $request->input('value');
-        $wiki = $request->attributes->get('wiki');
 
         WikiSetting::updateOrCreate(
             [
diff --git a/tests/Routes/Wiki/SettingUpdateTest.php b/tests/Routes/Wiki/SettingUpdateTest.php
index 5f9f1e4b..2808fcf0 100644
--- a/tests/Routes/Wiki/SettingUpdateTest.php
+++ b/tests/Routes/Wiki/SettingUpdateTest.php
@@ -102,8 +102,28 @@ public function testValidSetting($settingName, $settingValue, $expectedStored) {
         );
     }
 
+    public function testExistingModernDefaultCanBeRetained(): void {
+        $user = User::factory()->create(['verified' => true]);
+        $wiki = Wiki::factory('nodb')->create();
+        WikiManager::factory()->create(['wiki_id' => $wiki->id, 'user_id' => $user->id]);
+        WikiSetting::factory()->create([
+            'wiki_id' => $wiki->id,
+            'name' => 'wgDefaultSkin',
+            'value' => 'modern',
+        ]);
+
+        $this->actingAs($user, 'api')
+            ->json('POST', str_replace('foo', 'wgDefaultSkin', $this->route), [
+                'wiki' => $wiki->id,
+                'setting' => 'wgDefaultSkin',
+                'value' => 'modern',
+            ])
+            ->assertStatus(200);
+    }
+
     public static function provideValidSettingsBadValues() {
         yield ['wgDefaultSkin', 'foo'];
+        yield ['wgDefaultSkin', 'modern'];
         yield ['wwExtEnableConfirmAccount', 'foo'];
         yield ['wwWikibaseStringLengthString', 12];
         yield ['wwWikibaseStringLengthMonolingualText', 12];