diff --git a/app/Http/Controllers/ProductController.php b/app/Http/Controllers/ProductController.php
index 66fbd08..a32d15e 100644
--- a/app/Http/Controllers/ProductController.php
+++ b/app/Http/Controllers/ProductController.php
@@ -81,6 +81,14 @@ class ProductController extends Controller
             'large_unit' => 'nullable|string|max:50',
             'conversion_rate' => 'required_with:large_unit|nullable|numeric|min:0.0001',
             'purchase_unit' => 'nullable|string|max:50',
+        ], [
+            'name.required' => '商品名稱為必填',
+            'category_id.required' => '請選擇分類',
+            'category_id.exists' => '所選分類不存在',
+            'base_unit.required' => '基本庫存單位為必填',
+            'conversion_rate.required_with' => '填寫大單位時，換算率為必填',
+            'conversion_rate.numeric' => '換算率必須為數字',
+            'conversion_rate.min' => '換算率最小為 0.0001',
         ]);
 
         // Auto-generate code
@@ -109,7 +117,6 @@ class ProductController extends Controller
             'base_unit' => 'required|string|max:50',
             'large_unit' => 'nullable|string|max:50',
             'conversion_rate' => 'required_with:large_unit|nullable|numeric|min:0.0001',
-            'purchase_unit' => 'nullable|string|max:50',
         ]);
 
         $product->update($validated);
diff --git a/database/migrations/2025_12_31_172200_make_conversion_rate_nullable.php b/database/migrations/2025_12_31_172200_make_conversion_rate_nullable.php
new file mode 100644
index 0000000..bd6f2c0
--- /dev/null
+++ b/database/migrations/2025_12_31_172200_make_conversion_rate_nullable.php
@@ -0,0 +1,28 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('products', function (Blueprint $table) {
+            $table->decimal('conversion_rate', 10, 4)->nullable()->default(null)->change();
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('products', function (Blueprint $table) {
+            $table->decimal('conversion_rate', 10, 4)->nullable(false)->default(1)->change();
+        });
+    }
+};