diff --git a/.github/workflows/rails_application-coverage.yml b/.github/workflows/rails_application-coverage.yml
index 8f2ea509..3e9e41f2 100644
--- a/.github/workflows/rails_application-coverage.yml
+++ b/.github/workflows/rails_application-coverage.yml
@@ -9,10 +9,12 @@ on:
       - master
     paths:
       - "apps/rails_application/Gemfile.lock"
+      - "apps/rails_application/.mutant.yml"
       - ".github/workflows/rails_application-coverage.yml"
   pull_request:
     paths:
       - "apps/rails_application/Gemfile.lock"
+      - "apps/rails_application/.mutant.yml"
       - ".github/workflows/rails_application-coverage.yml"
   schedule:
     - cron: '0 17 * * *'
diff --git a/apps/rails_application/.mutant.yml b/apps/rails_application/.mutant.yml
index 29600ca5..97263c60 100644
--- a/apps/rails_application/.mutant.yml
+++ b/apps/rails_application/.mutant.yml
@@ -1,4 +1,6 @@
 ---
+environment_variables:
+  RAILS_ENV: test
 mutation:
   operators: light
   timeout: 10.0
diff --git a/apps/rails_application/Makefile b/apps/rails_application/Makefile
index 3898e74f..61d0ef99 100644
--- a/apps/rails_application/Makefile
+++ b/apps/rails_application/Makefile
@@ -6,9 +6,18 @@ install: ## Installs dependencies, runs migrations, creates db & seeds if necess
 dev:
 	@$(MAKE) -j 2 web css
 
-mutate: ## Run full mutation tests
+build-tailwind:
+	@echo "Building Tailwind CSS..."
+	@bin/rails tailwindcss:build
+
+mutate: build-tailwind
+ifdef MUTANT_SUBJECT
+	@echo "Running mutation tests for subject: '$(MUTANT_SUBJECT)'"
+	@bundle exec mutant run $(MUTANT_OPTS) -- '$(MUTANT_SUBJECT)'
+else
 	@echo "Running full mutation tests..."
-	@RAILS_ENV=test bundle exec mutant run
+	@bundle exec mutant run $(MUTANT_OPTS)
+endif
 
 mutate-changes: ## Run incremental mutation tests on changes
 	@echo "Running incremental mutation tests against $(GIT_BASE_SHA)..."
@@ -86,9 +95,7 @@ mutate-client-authentication: ## Run mutation tests for ClientAuthentication nam
 	@echo "Running mutation tests for ClientAuthentication..."
 	@RAILS_ENV=test bundle exec mutant run "ClientAuthentication*"
 
-test: ## Run unit tests
-	@echo "Building Tailwind CSS..."
-	@bin/rails tailwindcss:build
+test: build-tailwind
 	@echo "Running unit tests..."
 	@BROWSERSLIST_IGNORE_OLD_DATA=1 bin/rails test
 
@@ -101,9 +108,8 @@ css:
 web:
 	@bin/rails server -p 3000
 
-test-mutant-runner:
-	@bin/rails tailwindcss:build
+test-mutant-runner: build-tailwind
 	@RAILS_ENV=test bundle exec mutant test -j 4
 
-.PHONY: help install dev test mutate mutate-changes mutate-processes mutate-orders mutate-order-header mutate-client-orders mutate-invoices mutate-shipments mutate-vat-rates mutate-coupons mutate-products mutate-public-offer mutate-customers mutate-admin mutate-time-promotions mutate-returns mutate-availability mutate-client-inbox mutate-authorization mutate-client-authentication css web test-mutant-runner
+.PHONY: help install dev build-tailwind test mutate mutate-changes mutate-processes mutate-orders mutate-order-header mutate-client-orders mutate-invoices mutate-shipments mutate-vat-rates mutate-coupons mutate-products mutate-public-offer mutate-customers mutate-admin mutate-time-promotions mutate-returns mutate-availability mutate-client-inbox mutate-authorization mutate-client-authentication css web test-mutant-runner
 .DEFAULT_GOAL := help
diff --git a/apps/rails_application/test/invoices/assign_store_to_invoice_test.rb b/apps/rails_application/test/invoices/assign_store_to_invoice_test.rb
index 439deb91..4768a44b 100644
--- a/apps/rails_application/test/invoices/assign_store_to_invoice_test.rb
+++ b/apps/rails_application/test/invoices/assign_store_to_invoice_test.rb
@@ -91,5 +91,37 @@ def test_assigns_store_to_correct_invoice_when_multiple_invoices_exist
       assert_equal(store_id_1, invoice_1.store_id)
       assert_equal(store_id_2, invoice_2.store_id)
     end
+
+    def test_returns_nil_when_invoice_exists_in_different_store
+      event_store = Rails.configuration.event_store
+      store_id_1 = SecureRandom.uuid
+      store_id_2 = SecureRandom.uuid
+      invoice_id = SecureRandom.uuid
+
+      event_store.publish(
+        Invoicing::InvoiceItemAdded.new(
+          data: {
+            invoice_id: invoice_id,
+            product_id: SecureRandom.uuid,
+            title: "Test Product",
+            quantity: 1,
+            unit_price: 100,
+            vat_rate: { rate: 20, code: "VAT" }
+          }
+        )
+      )
+
+      event_store.publish(
+        Stores::InvoiceRegistered.new(
+          data: {
+            store_id: store_id_1,
+            invoice_id: invoice_id
+          }
+        )
+      )
+
+      assert_nil Invoices.find_invoice_in_store(invoice_id, store_id_2)
+      assert_not_nil Invoices.find_invoice_in_store(invoice_id, store_id_1)
+    end
   end
 end
diff --git a/apps/rails_application/test/test_helper.rb b/apps/rails_application/test/test_helper.rb
index 5f5bfb79..1bbc3e48 100644
--- a/apps/rails_application/test/test_helper.rb
+++ b/apps/rails_application/test/test_helper.rb
@@ -142,6 +142,7 @@ def before_setup
 
     Configuration.new.call(Rails.configuration.event_store, Rails.configuration.command_bus)
     @default_store_id = ensure_default_store
+    cookies[:current_store_id] = @default_store_id
     result
   end