diff --git a/core/lib/Drupal/Core/Entity/EntityManager.php b/core/lib/Drupal/Core/Entity/EntityManager.php
index 725ccbf..7a54289 100644
--- a/core/lib/Drupal/Core/Entity/EntityManager.php
+++ b/core/lib/Drupal/Core/Entity/EntityManager.php
@@ -577,7 +577,7 @@ public function getFieldMap() {
    * {@inheritdoc}
    */
   public function getFieldMapByFieldType($field_type) {
-    if (!array_key_exists($field_type, $this->fieldMapByFieldType)) {
+    if (!isset($this->fieldMapByFieldType[$field_type])) {
       $filtered_map = array();
       $map = $this->getFieldMap();
       foreach ($map as $entity_type => $fields) {
diff --git a/core/modules/comment/tests/src/CommentManagerTest.php b/core/modules/comment/tests/src/CommentManagerTest.php
index 77f874b..27c03a1 100644
--- a/core/modules/comment/tests/src/CommentManagerTest.php
+++ b/core/modules/comment/tests/src/CommentManagerTest.php
@@ -19,15 +19,22 @@ class CommentManagerTest extends UnitTestCase {
   /**
    * Tests the getFields method.
    *
-   * @see \Drupal\comment\CommentManager::getFields()
-   *
-   * @group Drupal
-   * @group Comment
+   * @covers ::getFields()
    */
   public function testGetFields() {
+    // Set up a content entity type.
+    $entity_type = $this->getMock('Drupal\Core\Entity\ContentEntityTypeInterface');
+    $entity_type->expects($this->any())
+      ->method('getClass')
+      ->will($this->returnValue('Node'));
+    $entity_type->expects($this->any())
+      ->method('isSubclassOf')
+      ->with('\Drupal\Core\Entity\ContentEntityInterface')
+      ->will($this->returnValue(TRUE));
+
     $entity_manager = $this->getMockBuilder('Drupal\Core\Entity\EntityManager')
       ->disableOriginalConstructor()
-      ->setMethods(array('getFieldMap'))
+      ->setMethods(array('getFieldMap', 'getDefinition'))
       ->getMock();
 
     $entity_manager->expects($this->once())
@@ -42,6 +49,11 @@ public function testGetFields() {
           ),
         ),
       )));
+
+    $entity_manager->expects($this->any())
+      ->method('getDefinition')
+      ->will($this->returnValue($entity_type));
+
     $comment_manager = new CommentManager(
       $entity_manager,
       $this->getMockBuilder('Drupal\Core\Entity\Query\QueryFactory')->disableOriginalConstructor()->getMock(),
@@ -56,8 +68,8 @@ public function testGetFields() {
     // Should not trigger EntityManager::getFieldMap().
     $cached_comment_fields = $comment_manager->getFields('node');
     $this->assertSame($cached_comment_fields, $comment_fields);
-    $this->assertArrayHasKey('field_foobar', $comment_fields['node']);
-    $this->assertArrayNotHasKey('field_fuzzball', $comment_fields['node']);
+    $this->assertArrayHasKey('field_foobar', $comment_fields);
+    $this->assertArrayNotHasKey('field_fuzzball', $comment_fields);
   }
 
 }