diff --git a/app/serializers/v1/invoices/billing_period_serializer.rb b/app/serializers/v1/invoices/billing_period_serializer.rb
index cc2252fe21d..6ee93aeeacd 100644
--- a/app/serializers/v1/invoices/billing_period_serializer.rb
+++ b/app/serializers/v1/invoices/billing_period_serializer.rb
@@ -12,6 +12,8 @@ def serialize
           subscription_to_datetime: model.to_datetime.iso8601,
           charges_from_datetime: model.charges_from_datetime.iso8601,
           charges_to_datetime: model.charges_to_datetime.iso8601,
+          fixed_charges_from_datetime: model.fixed_charges_from_datetime&.iso8601,
+          fixed_charges_to_datetime: model.fixed_charges_to_datetime&.iso8601,
           invoicing_reason: model.invoicing_reason
         }
       end
diff --git a/spec/factories/invoice_subscriptions.rb b/spec/factories/invoice_subscriptions.rb
index d39aaa1e82e..3d4a520a6f7 100644
--- a/spec/factories/invoice_subscriptions.rb
+++ b/spec/factories/invoice_subscriptions.rb
@@ -15,6 +15,8 @@
       to_datetime { timestamp.end_of_month }
       charges_from_datetime { from_datetime - 1.month }
       charges_to_datetime { to_datetime.end_of_month }
+      fixed_charges_from_datetime { from_datetime.beginning_of_month }
+      fixed_charges_to_datetime { to_datetime.end_of_month }
     end
   end
 end
diff --git a/spec/serializers/v1/invoices/billing_period_serializer_spec.rb b/spec/serializers/v1/invoices/billing_period_serializer_spec.rb
index cdfc48a5e03..4741851e2fc 100644
--- a/spec/serializers/v1/invoices/billing_period_serializer_spec.rb
+++ b/spec/serializers/v1/invoices/billing_period_serializer_spec.rb
@@ -10,15 +10,27 @@
   it "serializes the object" do
     result = JSON.parse(serializer.to_json)
 
-    aggregate_failures do
+    expect(result["billing_period"]["lago_subscription_id"]).to eq(invoice_subscription.subscription_id)
+    expect(result["billing_period"]["external_subscription_id"]).to eq(invoice_subscription.subscription.external_id)
+    expect(result["billing_period"]["lago_plan_id"]).to eq(invoice_subscription.subscription.plan_id)
+    expect(result["billing_period"]["subscription_from_datetime"]).to eq(invoice_subscription.from_datetime.iso8601)
+    expect(result["billing_period"]["subscription_to_datetime"]).to eq(invoice_subscription.to_datetime.iso8601)
+    expect(result["billing_period"]["charges_from_datetime"]).to eq(invoice_subscription.charges_from_datetime.iso8601)
+    expect(result["billing_period"]["charges_to_datetime"]).to eq(invoice_subscription.charges_to_datetime.iso8601)
+    expect(result["billing_period"]["fixed_charges_from_datetime"]).to eq(invoice_subscription.fixed_charges_from_datetime.iso8601)
+    expect(result["billing_period"]["fixed_charges_to_datetime"]).to eq(invoice_subscription.fixed_charges_to_datetime.iso8601)
+    expect(result["billing_period"]["invoicing_reason"]).to eq(invoice_subscription.invoicing_reason)
+  end
+
+  context "when legacy invoice subscription without fixed charges boundaries" do
+    let(:invoice_subscription) { build(:invoice_subscription, :boundaries, fixed_charges_from_datetime: nil, fixed_charges_to_datetime: nil) }
+
+    it "serializes the object without fixed charges boundaries" do
+      result = JSON.parse(serializer.to_json)
+
       expect(result["billing_period"]["lago_subscription_id"]).to eq(invoice_subscription.subscription_id)
-      expect(result["billing_period"]["external_subscription_id"]).to eq(invoice_subscription.subscription.external_id)
-      expect(result["billing_period"]["lago_plan_id"]).to eq(invoice_subscription.subscription.plan_id)
-      expect(result["billing_period"]["subscription_from_datetime"]).to eq(invoice_subscription.from_datetime.iso8601)
-      expect(result["billing_period"]["subscription_to_datetime"]).to eq(invoice_subscription.to_datetime.iso8601)
-      expect(result["billing_period"]["charges_from_datetime"]).to eq(invoice_subscription.charges_from_datetime.iso8601)
-      expect(result["billing_period"]["charges_to_datetime"]).to eq(invoice_subscription.charges_to_datetime.iso8601)
-      expect(result["billing_period"]["invoicing_reason"]).to eq(invoice_subscription.invoicing_reason)
+      expect(result["billing_period"]["fixed_charges_from_datetime"]).to be_nil
+      expect(result["billing_period"]["fixed_charges_to_datetime"]).to be_nil
     end
   end
 end