diff --git a/core/modules/migrate_drupal/src/Plugin/migrate/source/ContentEntity.php b/core/modules/migrate_drupal/src/Plugin/migrate/source/ContentEntity.php
index 4c2689b..ddf5629 100644
--- a/core/modules/migrate_drupal/src/Plugin/migrate/source/ContentEntity.php
+++ b/core/modules/migrate_drupal/src/Plugin/migrate/source/ContentEntity.php
@@ -1,12 +1,14 @@
 <?php
 
-namespace Drupal\migrate_drupal\Plugin\migrate\source\d8;
+namespace Drupal\migrate_drupal\Plugin\migrate\source;
 
 use Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException;
 use Drupal\Core\Entity\ContentEntityInterface;
+use Drupal\Core\Entity\ContentEntityTypeInterface;
 use Drupal\Core\Entity\EntityFieldManagerInterface;
 use Drupal\Core\Entity\EntityTypeManagerInterface;
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\migrate\MigrateException;
 use Drupal\migrate\Plugin\migrate\source\SourcePluginBase;
 use Drupal\migrate\Plugin\MigrationInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
@@ -17,7 +19,7 @@
  * @MigrateSource(
  *   id = "entity",
  *   source_module = "migrate_drupal",
- *   deriver = "\Drupal\migrate_drupal\Plugin\migrate\source\d8\ContentEntityDeriver",
+ *   deriver = "\Drupal\migrate_drupal\Plugin\migrate\source\ContentEntityDeriver",
  * )
  */
 class ContentEntity extends SourcePluginBase implements ContainerFactoryPluginInterface {
@@ -44,35 +46,27 @@ class ContentEntity extends SourcePluginBase implements ContainerFactoryPluginIn
   protected $bundleFields = [];
 
   /**
-   * The entity type.
+   * The entity definition.
    *
-   * @var string
+   * @var \Drupal\Core\Entity\EntityTypeInterface
    */
   protected $entityType;
 
   /**
-   * The entity key for a bundle or FALSE if not configured.
-   *
-   * @var bool|string
-   */
-  protected $bundleKey;
-
-  /**
    * {@inheritdoc}
    */
   public function __construct(array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration, EntityTypeManagerInterface $entity_type_manager, EntityFieldManagerInterface $entity_field_manager) {
     if (empty($plugin_definition['entity_type'])) {
-      throw new InvalidPluginDefinitionException($plugin_id, 'Missing required entity_type definition');
+      throw new InvalidPluginDefinitionException($plugin_id, 'Missing required "entity_type" definition');
     }
     $this->entityTypeManager = $entity_type_manager;
     $this->entityFieldManager = $entity_field_manager;
-    $this->entityType = $plugin_definition['entity_type'];
-    $entityDefinition = $this->entityTypeManager->getDefinition($this->entityType);
-    $this->bundleKey = $entityDefinition->getKey('bundle');
-    if (!empty($this->configuration['bundle'])) {
-      if (!$this->bundleKey) {
-        throw new InvalidPluginDefinitionException('A bundle was provided but entity is not bundleable.');
-      }
+    $this->entityType = $this->entityTypeManager->getDefinition($plugin_definition['entity_type']);
+    if (!$this->entityType instanceof ContentEntityTypeInterface) {
+      throw new InvalidPluginDefinitionException($plugin_id, 'The "entity" source plugin only supports content entities.');
+    }
+    if (!empty($this->configuration['bundle']) && !$this->entityType->getKey('bundle')) {
+      throw new InvalidPluginDefinitionException('A bundle was provided but the entity type is not bundleable.');
     }
     parent::__construct($configuration + ['bundle' => NULL], $plugin_id, $plugin_definition, $migration);
   }
@@ -95,7 +89,7 @@ public static function create(ContainerInterface $container, array $configuratio
    * {@inheritdoc}
    */
   public function __toString() {
-    return $this->entityType;
+    return (string) sprintf('%s source', $this->entityType->id());
   }
 
   /**
@@ -106,7 +100,7 @@ public function __toString() {
    */
   protected function initializeIterator() {
     $ids = $this->query()->execute();
-    return $this->yield($ids);
+    return $this->yieldEntities($ids);
   }
 
   /**
@@ -118,8 +112,9 @@ protected function initializeIterator() {
    * @return \Generator
    *   And iterable of loaded entities.
    */
-  protected function yield(array $ids) {
-    $storage = $this->entityTypeManager->getStorage($this->entityType);
+  protected function yieldEntities(array $ids) {
+    $storage = $this->entityTypeManager
+      ->getStorage($this->entityType->id());
     foreach ($ids as $id) {
       /** @var \Drupal\Core\Entity\ContentEntityInterface $entity */
       $entity = $storage->load($id);
@@ -133,6 +128,9 @@ protected function yield(array $ids) {
   /**
    * Convert entity to array.
    *
+   * Make all IDs flat values. All other values are returned as per
+   * $entity->toArray(), which is a nested array.
+   *
    * @param \Drupal\Core\Entity\ContentEntityInterface $entity
    *   The entity to convert.
    *
@@ -140,14 +138,17 @@ protected function yield(array $ids) {
    *   The entity converted to array.
    */
   protected function toArray(ContentEntityInterface $entity) {
-    $return = [];
-    foreach ($entity->toArray() as $field => $values) {
-      $return[$field] = $values;
-    }
-    // The ids must be flat, they cannot be nested for the id map.
+    $return = $entity->toArray();
+    // This is necessary because the IDs must be flat. They cannot be nested for
+    // the ID map.
     foreach (array_keys($this->getIds()) as $id) {
       $reset = reset($return[$id]);
+      // Force the IDs over top the previous values.
+      unset($return[$id]);
       $return[$id] = reset($reset);
+      if (empty($return[$id])) {
+        throw new MigrateException(sprintf('Entity of type %s and id %s is missing its id.', $entity->getEntityType(), $entity->id()));
+      }
     }
     return $return;
   }
@@ -156,35 +157,39 @@ protected function toArray(ContentEntityInterface $entity) {
    * Query to retrieve entities.
    *
    * @return \Drupal\Core\Entity\Query\QueryInterface
+   *   The query.
    */
   public function query() {
     $query = $this->entityTypeManager
-      ->getStorage($this->entityType)
+      ->getStorage($this->entityType->id())
       ->getQuery();
     if (!empty($this->configuration['bundle'])) {
-      $query->condition($this->bundleKey, $this->configuration['bundle']);
+      $query->condition($this->entityType->getKey('bundle'), $this->configuration['bundle']);
     }
     return $query;
   }
 
+
   /**
    * {@inheritdoc}
    */
-  protected function doCount() {
-    return $this->query()->count()->execute();
+  public function count($refresh = FALSE) {
+    // @TODO: Figure out a better way to retrieve a valid count.
+    // https://www.drupal.org/project/drupal/issues/2937166
+    return -1;
   }
 
   /**
    * {@inheritdoc}
    */
   public function fields() {
-    $field_definitions = $this->entityFieldManager->getBaseFieldDefinitions($this->entityType);
+    $field_definitions = $this->entityFieldManager->getBaseFieldDefinitions($this->entityType->id());
     $fields = [];
     foreach ($field_definitions as $field_name => $definition) {
       $fields[$field_name] = (string) $definition->getLabel();
     }
     if (!empty($this->configuration['bundle'])) {
-      foreach ($this->entityFieldManager->getFieldDefinitions($this->entityType, $this->configuration['bundle']) as $field_name => $definition) {
+      foreach ($this->entityFieldManager->getFieldDefinitions($this->entityType->id(), $this->configuration['bundle']) as $field_name => $definition) {
         $fields[$field_name] = (string) $definition->getLabel();
       }
     }
@@ -195,11 +200,10 @@ public function fields() {
    * {@inheritdoc}
    */
   public function getIds() {
-    $entity_definition = $this->entityTypeManager->getDefinition($this->entityType);
-    $idKey = $entity_definition->getKey('id');
-    $ids[$idKey] = $this->getDefinitionFromEntity($idKey);
-    if ($entity_definition->isTranslatable()) {
-      $langcode_key = $entity_definition->getKey('langcode');
+    $id_key = $this->entityType->getKey('id');
+    $ids[$id_key] = $this->getDefinitionFromEntity($id_key);
+    if ($this->entityType->isTranslatable()) {
+      $langcode_key = $this->entityType->getKey('langcode');
       $ids[$langcode_key] = $this->getDefinitionFromEntity($langcode_key);
     }
     return $ids;
@@ -220,7 +224,7 @@ public function getIds() {
    */
   protected function getDefinitionFromEntity($key) {
     /** @var \Drupal\Core\Field\FieldDefinitionInterface $field_definition */
-    $field_definition = $this->entityFieldManager->getBaseFieldDefinitions($this->entityType)[$key];
+    $field_definition = $this->entityFieldManager->getBaseFieldDefinitions($this->entityType->id())[$key];
     return [
       'alias' => 'b',
       'type' => $field_definition->getType(),
diff --git a/core/modules/migrate_drupal/src/Plugin/migrate/source/ContentEntityDeriver.php b/core/modules/migrate_drupal/src/Plugin/migrate/source/ContentEntityDeriver.php
index 33052be..38050ca 100644
--- a/core/modules/migrate_drupal/src/Plugin/migrate/source/ContentEntityDeriver.php
+++ b/core/modules/migrate_drupal/src/Plugin/migrate/source/ContentEntityDeriver.php
@@ -1,6 +1,6 @@
 <?php
 
-namespace Drupal\migrate_drupal\Plugin\migrate\source\d8;
+namespace Drupal\migrate_drupal\Plugin\migrate\source;
 
 use Drupal\Component\Plugin\Derivative\DeriverBase;
 use Drupal\Core\Entity\ContentEntityTypeInterface;
diff --git a/core/modules/migrate_drupal/tests/src/Kernel/Plugin/migrate/source/ContentEntityTest.php b/core/modules/migrate_drupal/tests/src/Kernel/Plugin/migrate/source/ContentEntityTest.php
index f659481..f290c19 100755
--- a/core/modules/migrate_drupal/tests/src/Kernel/Plugin/migrate/source/ContentEntityTest.php
+++ b/core/modules/migrate_drupal/tests/src/Kernel/Plugin/migrate/source/ContentEntityTest.php
@@ -1,6 +1,6 @@
 <?php
 
-namespace Drupal\Tests\migrate_drupal\Kernel\Plugin\migrate\source\d8;
+namespace Drupal\Tests\migrate_drupal\Kernel\Plugin\migrate\source;
 
 use Drupal\Core\Field\FieldStorageDefinitionInterface;
 use Drupal\Core\Language\LanguageInterface;
@@ -187,6 +187,7 @@ public function testUserSource() {
     $userSource = $this->sourcePluginManager->createInstance('entity:user', [], $this->migration);
     $this->migration->expects(self::once())->method('getSourcePlugin')->willReturn($userSource);
     $this->migration->expects(self::once())->method('getDestinationIds')->willReturn([1]);
+    $this->assertSame('user source', $userSource->__toString());
     $ids = $userSource->getIds();
     $this->assertArrayHasKey('langcode', $ids);
     $this->assertArrayHasKey('uid', $ids);
@@ -218,6 +219,7 @@ public function testFileSource() {
     $fileSource = $this->sourcePluginManager->createInstance('entity:file', [], $this->migration);
     $this->migration->expects(self::once())->method('getSourcePlugin')->willReturn($fileSource);
     $this->migration->expects(self::once())->method('getDestinationIds')->willReturn([1]);
+    $this->assertSame('file source', $fileSource->__toString());
     $ids = $fileSource->getIds();
     $this->assertArrayHasKey('fid', $ids);
     $fields = $fileSource->fields();
@@ -242,6 +244,7 @@ public function testNodeSource() {
     $this->migration->expects(self::exactly(2))->method('getSourcePlugin')->willReturn($nodeSource);
     $this->migration->expects(self::at(0))->method('getDestinationIds')->willReturn([1, 'en']);
     $this->migration->expects(self::at(1))->method('getDestinationIds')->willReturn([2, 'fr']);
+    $this->assertSame('node source', $nodeSource->__toString());
     $ids = $nodeSource->getIds();
     $this->assertArrayHasKey('langcode', $ids);
     $this->assertArrayHasKey('nid', $ids);
@@ -286,6 +289,7 @@ public function testMediaSource() {
     $mediaSource = $this->sourcePluginManager->createInstance('entity:media', ['bundle' => 'image'], $this->migration);
     $this->migration->expects(self::once())->method('getSourcePlugin')->willReturn($mediaSource);
     $this->migration->expects(self::once())->method('getDestinationIds')->willReturn([1]);
+    $this->assertSame('media source', $mediaSource->__toString());
     $ids = $mediaSource->getIds();
     $this->assertArrayHasKey('langcode', $ids);
     $this->assertArrayHasKey('mid', $ids);
@@ -319,6 +323,7 @@ public function testTermSource() {
     $this->migration->expects(self::exactly(2))->method('getSourcePlugin')->willReturn($termSource);
     $this->migration->expects(self::at(0))->method('getDestinationIds')->willReturn([1]);
     $this->migration->expects(self::at(1))->method('getDestinationIds')->willReturn([2]);
+    $this->assertSame('taxonomy_term source', $termSource->__toString());
     $ids = $termSource->getIds();
     $this->assertArrayHasKey('langcode', $ids);
     $this->assertArrayHasKey('tid', $ids);