Potential fix for #4180

Author: lukeholder

src/base/Purchasable.php

@@ -902,12 +902,22 @@ function($attribute, $params, Validator $validator) use ($lineItem, $lineItemQua
      */
     public function setAttributes($values, $safeOnly = true): void
     {
-        // Normalize empty strings to null for category IDs before setting
-        if (isset($values['taxCategoryId']) && $values['taxCategoryId'] === '') {
-            $values['taxCategoryId'] = null;
+        // Normalize category IDs - handle arrays from componentSelect and empty strings
+        if (isset($values['taxCategoryId'])) {
+            if (is_array($values['taxCategoryId'])) {
+                $values['taxCategoryId'] = reset($values['taxCategoryId']) ?: null;
+            }
+            if ($values['taxCategoryId'] === '') {
+                $values['taxCategoryId'] = null;
+            }
         }
-        if (isset($values['shippingCategoryId']) && $values['shippingCategoryId'] === '') {
-            $values['shippingCategoryId'] = null;
+        if (isset($values['shippingCategoryId'])) {
+            if (is_array($values['shippingCategoryId'])) {
+                $values['shippingCategoryId'] = reset($values['shippingCategoryId']) ?: null;
+            }
+            if ($values['shippingCategoryId'] === '') {
+                $values['shippingCategoryId'] = null;
+            }
         }
 
         parent::setAttributes($values, $safeOnly);