NoReverseMatch を解決したい。

前提・実現したいこと

DjangoでECアプリの機能の実装中に以下のエラーメッセージが発生しました。

発生している問題・エラーメッセージ

NoReverseMatch at /ecapp/product/9/
Reverse for 'detail' with arguments '('',)' not found. 1 pattern(s) tried: ['ecapp/product/(?P<product_id>[0-9]+)/$']

Request Method:	POST
Request URL:	http://127.0.0.1:8000/ecapp/product/9/
Django Version:	2.2.5
Exception Type:	NoReverseMatch
Exception Value:	
Reverse for 'detail' with arguments '('',)' not found. 1 pattern(s) tried: ['ecapp/product/(?P<product_id>[0-9]+)/$']

該当のソースコード

detail.html

html
1<form action="{% url 'ecapp:detail' product.id %}" method="post">{% csrf_token %}
2    {{ add_to_cart_form.as_p }}
3    <button type="submit" name="buy_now" class="btn btn-primary"> 今すぐ購入 </button>
4    <button type="submit" name="in_cart" class="btn btn-outline-primary">
5    <span class="text">カートに入れる</span> <i class="fas fa-shopping-cart"></i>
6    </button>
7</form>

urls.py

python
1app_name = 'ecapp'
2urlpatterns = [
3    path('product/<int:product_id>/', views.detail, name='detail'),
4    path('cart/', views.cart, name='cart'),
5]

views.py

python
1def detail(request, product_id):
2    product = get_object_or_404(Product, pk=product_id)
3    if request.method == 'POST':
4        num = request.POST['num']
5        num = int(num)
6        if num == 0:
7            messages.warning(request, '個数を選択してください')
8            return redirect('ecapp:detail', product_id=product_id)
9        # カートに追加されたとき
10        if 'in_cart' in request.POST:
11            if 'cart' in request.session:
12                if str(product_id) in request.session['cart']:
13                    request.session['cart'][str(product_id)] += num
14                else:
15                    request.session['cart'][str(product_id)] = num
16            else:
17                request.session['cart'] = {str(product_id): num}
18            messages.success(request, f"{product.name}を{num}個カートに入れました！")
19            return redirect('ecapp:cart')
20        # 今すぐ購入されたとき
21        elif 'buy_now' in request.POST:
22            cart_products = {str(product_id): num}
23            purchase_form = ProfileForm(request.POST, None)
24            total_price = product.price * num
25            context = {
26                'purchase_form': purchase_form,
27                'cart_products': cart_products,
28                'total_price': total_price,
29            }
30            return render(request, 'ecapp/cart.html', context)
31
32    else:
33        add_to_cart_form = AddToCartForm(product_id=product_id)
34        context = {
35            'product': product,
36            'add_to_cart_form': add_to_cart_form,
37        }
38    return render(request, 'ecapp/detail.html', context)