diff -u b/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php b/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php
--- b/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php
+++ b/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php
@@ -10,6 +10,7 @@
 use Drupal\Component\Plugin\ConfigurablePluginInterface;
 use Drupal\Component\Utility\String;
 use Drupal\Core\Cache\Cache;
+use Drupal\Core\Config\Schema\SchemaIncompleteException;
 use Drupal\Core\Entity\Entity;
 use Drupal\Core\Config\ConfigDuplicateUUIDException;
 use Drupal\Core\Entity\EntityStorageInterface;
@@ -243,22 +244,13 @@
     $properties = array();
     $config_name = $this->getEntityType()->getConfigPrefix() . '.' . $this->id();
     $definition = $this->getTypedConfig()->getDefinition($config_name);
-    if (isset($definition['mapping'])) {
-      $keys = array_keys($definition['mapping']);
-    }
-    else {
-      // Some configuration objects do not have a schema. Extract key names from
-      // class properties.
-      // @todo Remove once migration config entities have schema
-      //   https://drupal.org/node/2183957.
-      $class_info = new \ReflectionClass($this);
-      $keys = array();
-      foreach ($class_info->getProperties(\ReflectionProperty::IS_PUBLIC) as $property) {
-        $keys[] = $property->getName();
-      }
+    if (!isset($definition['mapping'])) {
+      throw new SchemaIncompleteException(String::format('Incomplete or missing schema for @config_name', array('@config_name' => $config_name)));
     }
     $id_key = $this->getEntityType()->getKey('id');
-    foreach ($keys as $name) {
+    foreach (array_keys($definition['mapping']) as $name) {
+      // Special handling for IDs so that computed compound IDs work.
+      // @see \Drupal\entity\EntityDisplayBase::id()
       if ($name == $id_key) {
         $properties[$name] = $this->id();
       }
diff -u b/core/tests/Drupal/Tests/Core/Config/Entity/ConfigEntityBaseUnitTest.php b/core/tests/Drupal/Tests/Core/Config/Entity/ConfigEntityBaseUnitTest.php
--- b/core/tests/Drupal/Tests/Core/Config/Entity/ConfigEntityBaseUnitTest.php
+++ b/core/tests/Drupal/Tests/Core/Config/Entity/ConfigEntityBaseUnitTest.php
@@ -451,17 +451,11 @@
 
   /**
    * @covers ::toArray
+   *
+   * @expectedException \Drupal\Core\Config\Schema\SchemaIncompleteException
    */
   public function testToArrayFallback() {
-    $properties = $this->entity->toArray();
-    $this->assertInternalType('array', $properties);
-    $class_info = new \ReflectionClass($this->entity);
-    foreach ($class_info->getProperties(\ReflectionProperty::IS_PUBLIC) as $property) {
-      $name = $property->getName();
-      $this->assertArrayHasKey($name, $properties);
-      $this->assertSame($this->entity->get($name), $properties[$name]);
-    }
-    $this->assertArrayNotHasKey('dependencies', $properties);
+    $this->entity->toArray();
   }
 
 }
diff -u b/core/tests/Drupal/Tests/Core/Config/Entity/ConfigEntityStorageTest.php b/core/tests/Drupal/Tests/Core/Config/Entity/ConfigEntityStorageTest.php
--- b/core/tests/Drupal/Tests/Core/Config/Entity/ConfigEntityStorageTest.php
+++ b/core/tests/Drupal/Tests/Core/Config/Entity/ConfigEntityStorageTest.php
@@ -175,7 +175,9 @@
     $this->cacheBackend = $this->getMock('Drupal\Core\Cache\CacheBackendInterface');
 
     $this->typedConfigManager = $this->getMock('Drupal\Core\Config\TypedConfigManagerInterface');
-
+    $this->typedConfigManager->expects($this->any())
+      ->method('getDefinition')
+      ->will($this->returnValue(array('mapping' => array('id' => '', 'uuid' => '', 'dependencies' => ''))));
     $container = new ContainerBuilder();
     $container->set('entity.manager', $this->entityManager);
     $container->set('config.typed', $this->typedConfigManager);
only in patch2:
unchanged:
--- a/core/modules/migrate/lib/Drupal/migrate/Entity/Migration.php
+++ b/core/modules/migrate/lib/Drupal/migrate/Entity/Migration.php
@@ -339,4 +339,18 @@ public function checkRequirements() {
     return TRUE;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function toArray() {
+    // @todo Remove once migration config entities have schema
+    //   https://drupal.org/node/2183957.
+    $class_info = new \ReflectionClass($this);
+    foreach ($class_info->getProperties(\ReflectionProperty::IS_PUBLIC) as $property) {
+      $name = $property->getName();
+      $properties[$name] = $this->get($name);
+    }
+    return $properties;
+  }
+
 }