@@ -0,0 +1,19 @@
+<?php
+
+namespace Tests\FrameworkBundle\Unit\Model\Customer;
+
+use PHPUnit\Framework\TestCase;
+use Shopsys\FrameworkBundle\Model\Customer\User\CustomerUserIdentifier;
+use Shopsys\FrameworkBundle\Model\Customer\Exception\EmptyCustomerUserIdentifierException;
+
+class CustomerUserIdentifierTest extends TestCase
+{
+    public function testCannotCreateIdentifierForEmptyCartIdentifierAndNullUser()
+    {
+        $cartIdentifier = '';
+        $customerUser = null;
+
+        $this->expectException(EmptyCustomerUserIdentifierException::class);
+        new CustomerUserIdentifier($cartIdentifier, $customerUser);
+    }
+}
I would rename it to `testCannotCreateIdentifierForEmptyCartIdentifierAndNullCustomerUser`