diff --git a/app/Http/Requests/UpdateAssetRequest.php b/app/Http/Requests/UpdateAssetRequest.php
index 1b379358f9..6d01dc5a30 100644
--- a/app/Http/Requests/UpdateAssetRequest.php
+++ b/app/Http/Requests/UpdateAssetRequest.php
@@ -28,23 +28,31 @@ class UpdateAssetRequest extends ImageUploadRequest
      */
     public function rules()
     {
+        $setting = Setting::getSettings();
+
         $rules = array_merge(
             parent::rules(),
             (new Asset)->getRules(),
-            // this is to overwrite rulesets that include required, and rewrite unique_undeleted
+            // This overwrites the rulesets that are set at the model level (via Watson) but are not necessarily required at the request level when doing a PATCH update.
+            // Confusingly, this skips the unique_undeleted validator at the model level (and therefore the UniqueUndeletedTrait), so we have to re-add those
+            // rules here without the requiredness, since those values will already exist if you're updating an existing asset.
             [
                 'model_id'  => ['integer', 'exists:models,id,deleted_at,NULL', 'not_array'],
                 'status_id' => ['integer', 'exists:status_labels,id'],
                 'asset_tag' => [
                     'min:1', 'max:255', 'not_array',
-                    Rule::unique('assets', 'asset_tag')->ignore($this->asset)->withoutTrashed()
+                    Rule::unique('assets', 'asset_tag')->ignore($this->asset)->withoutTrashed(),
+                ],
+                'serial' => [
+                    'string', 'max:255', 'not_array',
+                    $setting->unique_serial=='1' ? Rule::unique('assets', 'serial')->ignore($this->asset)->withoutTrashed() : 'nullable',
                 ],
             ],
         );
 
         // if the purchase cost is passed in as a string **and** the digit_separator is ',' (as is common in the EU)
         // then we tweak the purchase_cost rule to make it a string
-        if (Setting::getSettings()->digit_separator === '1.234,56' && is_string($this->input('purchase_cost'))) {
+        if ($setting->digit_separator === '1.234,56' && is_string($this->input('purchase_cost'))) {
             $rules['purchase_cost'] = ['nullable', 'string'];
         }