diff --git a/src/Routing/Routes.php b/src/Routing/Routes.php
index d54a5ca..db22c8f 100644
--- a/src/Routing/Routes.php
+++ b/src/Routing/Routes.php
@@ -116,8 +116,8 @@ class Routes implements ContainerInjectionInterface {
 
       // Collection endpoint, like /jsonapi/file/photo.
       $route_collection = (new Route($route_base_path, $defaults))
-        ->setRequirement('_entity_type', $resource_type->getEntityTypeId())
-        ->setRequirement('_bundle', $resource_type->getBundle())
+        ->setRequirement('_entity_type', (string) $resource_type->getEntityTypeId())
+        ->setRequirement('_bundle', (string) $resource_type->getBundle())
         ->setRequirement('_permission', 'access content')
         ->setRequirement('_jsonapi_custom_query_parameter_names', 'TRUE')
         ->setOption('serialization_class', JsonApiDocumentTopLevel::class)
@@ -129,8 +129,8 @@ class Routes implements ContainerInjectionInterface {
       $parameters = [$resource_type->getEntityTypeId() => ['type' => 'entity:' . $resource_type->getEntityTypeId()]];
       $route_individual = (new Route(sprintf('%s/{%s}', $route_base_path, $resource_type->getEntityTypeId())))
         ->addDefaults($defaults)
-        ->setRequirement('_entity_type', $resource_type->getEntityTypeId())
-        ->setRequirement('_bundle', $resource_type->getBundle())
+        ->setRequirement('_entity_type', (string) $resource_type->getEntityTypeId())
+        ->setRequirement('_bundle', (string) $resource_type->getBundle())
         ->setRequirement('_permission', 'access content')
         ->setRequirement('_jsonapi_custom_query_parameter_names', 'TRUE')
         ->setOption('parameters', $parameters)
@@ -142,8 +142,8 @@ class Routes implements ContainerInjectionInterface {
 
       // Related resource, like /jsonapi/file/photo/123/comments.
       $route_related = (new Route(sprintf('%s/{%s}/{related}', $route_base_path, $resource_type->getEntityTypeId()), $defaults))
-        ->setRequirement('_entity_type', $resource_type->getEntityTypeId())
-        ->setRequirement('_bundle', $resource_type->getBundle())
+        ->setRequirement('_entity_type', (string) $resource_type->getEntityTypeId())
+        ->setRequirement('_bundle', (string) $resource_type->getBundle())
         ->setRequirement('_permission', 'access content')
         ->setRequirement('_jsonapi_custom_query_parameter_names', 'TRUE')
         ->setOption('parameters', $parameters)
@@ -154,8 +154,8 @@ class Routes implements ContainerInjectionInterface {
 
       // Related endpoint, like /jsonapi/file/photo/123/relationships/comments.
       $route_relationship = (new Route(sprintf('%s/{%s}/relationships/{related}', $route_base_path, $resource_type->getEntityTypeId()), $defaults + ['_on_relationship' => TRUE]))
-        ->setRequirement('_entity_type', $resource_type->getEntityTypeId())
-        ->setRequirement('_bundle', $resource_type->getBundle())
+        ->setRequirement('_entity_type', (string) $resource_type->getEntityTypeId())
+        ->setRequirement('_bundle', (string) $resource_type->getBundle())
         ->setRequirement('_permission', 'access content')
         ->setRequirement('_jsonapi_custom_query_parameter_names', 'TRUE')
         ->setOption('parameters', $parameters)
diff --git a/tests/src/Unit/Routing/RoutesTest.php b/tests/src/Unit/Routing/RoutesTest.php
index cb1285d..f6b2899 100644
--- a/tests/src/Unit/Routing/RoutesTest.php
+++ b/tests/src/Unit/Routing/RoutesTest.php
@@ -34,6 +34,7 @@ class RoutesTest extends UnitTestCase {
     $resource_type_repository = $this->prophesize(ResourceTypeRepository::class);
     $resource_type_repository->all()->willReturn([
       new ResourceType('entity_type_1', 'bundle_1_1', EntityInterface::class),
+      new ResourceType('entity_type_2', 123, EntityInterface::class),
     ]);
     $container = $this->prophesize(ContainerInterface::class);
     $container->get('jsonapi.resource_type.repository')->willReturn($resource_type_repository->reveal());
@@ -54,8 +55,8 @@ class RoutesTest extends UnitTestCase {
     // Get the route collection and start making assertions.
     $routes = $this->routes['ok']->routes();
 
-    // Make sure that there are 4 routes for each resource.
-    $this->assertEquals(4, $routes->count());
+    // Make sure that there are 4 routes for each resource (2*4).
+    $this->assertEquals(8, $routes->count());
 
     $iterator = $routes->getIterator();
     // Check the collection route.