diff --git a/app/models/order_item.rb b/app/models/order_item.rb
index ee32c6c..c7346d8 100644
--- a/app/models/order_item.rb
+++ b/app/models/order_item.rb
@@ -8,7 +8,11 @@ class OrderItem < ApplicationRecord
   # una modificación que tenga que devolverle unidades no sabe a dónde.
   belongs_to :warehouse, optional: true
 
-  validates :quantity, numericality: { greater_than: 0 }
+  # Entero: la columna es integer y guardaba `0.5` como 0 y `2.7` como 2. El
+  # primero pasaba la validación (0,5 > 0) y reventaba después en DeductStock
+  # con un 500; el segundo se truncaba sin aviso. Se valida antes de escribir,
+  # así el alta y el webhook responden 422 con el motivo.
+  validates :quantity, numericality: { only_integer: true, greater_than: 0 }
   validates :unit_price, numericality: { greater_than_or_equal_to: 0 }
   validate :product_belongs_to_same_company_as_order
   validate :warehouse_belongs_to_same_company_as_order
diff --git a/spec/models/order_item_spec.rb b/spec/models/order_item_spec.rb
index d38b3ab..aa02c12 100644
--- a/spec/models/order_item_spec.rb
+++ b/spec/models/order_item_spec.rb
@@ -40,6 +40,19 @@
     expect(order_item).not_to be_valid
   end
 
+  # La columna es integer: 0,5 se guardaba como 0 y 2,7 como 2.
+  it 'refuses a fractional quantity', :aggregate_failures do
+    [0.5, 2.7, '1.5'].each do |quantity|
+      order_item.quantity = quantity
+      expect(order_item).not_to be_valid, "#{quantity.inspect} should be refused"
+    end
+  end
+
+  it 'accepts an integer that comes as text from a form' do
+    order_item.quantity = '3'
+    expect(order_item).to be_valid
+  end
+
   it 'validates unit_price is not negative' do
     order_item.unit_price = -1
     expect(order_item).not_to be_valid
diff --git a/spec/requests/api/v1/orders_spec.rb b/spec/requests/api/v1/orders_spec.rb
index 63f3ddb..3cafee1 100644
--- a/spec/requests/api/v1/orders_spec.rb
+++ b/spec/requests/api/v1/orders_spec.rb
@@ -396,6 +396,21 @@ def order_of_another_company
         expect(response).to have_http_status(:unprocessable_content)
       end
 
+      # Pasaba la validación (0,5 > 0), la columna integer la guardaba en 0 y
+      # DeductStock reventaba con ArgumentError: 500.
+      it 'rejects a quantity below one unit with 422, not 500', :aggregate_failures do
+        post_order(build_payload(items: [default_item.merge(quantity: 0.5)]))
+
+        expect(response).to have_http_status(:unprocessable_content)
+        expect(response.parsed_body['error']).to include('must be an integer')
+      end
+
+      it 'rejects a fractional quantity instead of truncating it', :aggregate_failures do
+        expect { post_order(build_payload(items: [default_item.merge(quantity: 2.7)])) }
+          .not_to change(Order, :count)
+        expect(response).to have_http_status(:unprocessable_content)
+      end
+
       it 'rejects when warehouse belongs to another company' do
         other_wh = other_company_warehouse
         post_order(build_payload(items: [default_item.merge(warehouse_id: other_wh.id)]))