diff --git a/tests/src/Functional/InternalEntitiesTest.php b/tests/src/Functional/InternalEntitiesTest.php
index 9a0ac98..aa3085e 100644
--- a/tests/src/Functional/InternalEntitiesTest.php
+++ b/tests/src/Functional/InternalEntitiesTest.php
@@ -38,6 +38,13 @@ class InternalEntitiesTest extends BrowserTestBase {
    */
   protected $testUser;
 
+  /**
+   * An entity of an internal entity type.
+   *
+   * @var \Drupal\Core\Entity\EntityInterface
+   */
+  protected $internalEntity;
+
   /**
    * An entity referencing an internal entity.
    *
@@ -66,16 +73,49 @@ class InternalEntitiesTest extends BrowserTestBase {
       'Internal Entities',
       'jsonapi_entity_test_no_label'
     );
-    $internal_entity = EntityTestNoLabel::create([]);
-    $internal_entity->save();
+    $this->internalEntity = EntityTestNoLabel::create([]);
+    $this->internalEntity->save();
     $this->referencingEntity = EntityTestWithBundle::create([
       'type' => 'internal_referencer',
-      'field_internal' => $internal_entity->id(),
+      'field_internal' => $this->internalEntity->id(),
     ]);
     $this->referencingEntity->save();
     drupal_flush_all_caches();
   }
 
+  /**
+   * Ensures that internal resources types aren't present in the routes.
+   */
+  public function testRoutes() {
+    if (!method_exists(EntityTypeInterface::class, 'isInternal')) {
+      $this->markTestSkipped('The Drupal Core version must be >= 8.5');
+      return;
+    }
+    $this->drupalLogin($this->testUser);
+    $internal_entity_type_id = $this->internalEntity->getEntityTypeId();
+    $internal_bundle = $this->internalEntity->bundle();
+    $internal_uuid = $this->internalEntity->uuid();
+    $referencing_entity_type_id = $this->referencingEntity->getEntityTypeId();
+    $referencing_bundle = $this->referencingEntity->bundle();
+    $referencing_uuid = $this->referencingEntity->uuid();
+    // This cannot be in a data provider because it needs values created by the
+    // setUp method.
+    $paths = [
+      'individual' => "/jsonapi/{$internal_entity_type_id}/{$internal_bundle}/{$internal_uuid}",
+      'collection' => "/jsonapi/{$internal_entity_type_id}/{$internal_bundle}",
+      'related' => "/jsonapi/{$referencing_entity_type_id}/{$referencing_bundle}/{$referencing_uuid}/field_internal",
+    ];
+    foreach ($paths as $type => $path) {
+      $response = $this->drupalGet($path, [], ['Accept' => 'application/vnd.api+json']);
+      $decoded = Json::decode($response);
+      $this->assertEquals(
+        404,
+        $decoded['errors'][0]['status'],
+        "The '{$type}' route ({$path}) should not be available for internal resource types.'"
+      );
+    }
+  }
+
   /**
    * Asserts that internal entities are not included in compound documents.
    */
@@ -88,7 +128,7 @@ class InternalEntitiesTest extends BrowserTestBase {
     $entity_type_id = $this->referencingEntity->getEntityTypeId();
     $bundle = $this->referencingEntity->bundle();
     $path = "/jsonapi/{$entity_type_id}/{$bundle}/{$this->referencingEntity->uuid()}?include=field_internal";
-    $response = $this->drupalGet($path);
+    $response = $this->drupalGet($path, [], ['Accept' => 'application/vnd.api+json']);
     $decoded = Json::decode($response);
     $this->assertArrayNotHasKey(
       'included',
diff --git a/tests/src/Kernel/Controller/EntityResourceTest.php b/tests/src/Kernel/Controller/EntityResourceTest.php
index 950d5da..b753dd0 100644
--- a/tests/src/Kernel/Controller/EntityResourceTest.php
+++ b/tests/src/Kernel/Controller/EntityResourceTest.php
@@ -943,6 +943,10 @@ class EntityResourceTest extends JsonapiKernelTestBase {
    *   The entity type ID.
    * @param string $bundle
    *   The bundle.
+   * @param array $relatable_resource_types
+   *   An array of relatable resource types, keyed by field.
+   * @param bool $internal
+   *   Whether the primary resource type is internal.
    *
    * @return \Drupal\jsonapi\Controller\EntityResource
    *   The resource.