diff --git a/lib/Doctrine/ORM/Mapping/ClassMetadataInfo.php b/lib/Doctrine/ORM/Mapping/ClassMetadataInfo.php
index b5eebcdde..5fa349933 100644
--- a/lib/Doctrine/ORM/Mapping/ClassMetadataInfo.php
+++ b/lib/Doctrine/ORM/Mapping/ClassMetadataInfo.php
@@ -29,6 +29,7 @@ use ReflectionClass;
 use Doctrine\Common\Persistence\Mapping\ClassMetadata;
 use Doctrine\Common\ClassLoader;
 use Doctrine\ORM\Cache\CacheException;
+use Doctrine\ORM\Cache\AssociationCacheEntry;
 
 /**
  * A <tt>ClassMetadata</tt> instance holds all the object-relational mapping metadata
@@ -718,7 +719,7 @@ class ClassMetadataInfo implements ClassMetadata
             $id = [];
 
             foreach ($this->identifier as $idField) {
-                $value = $this->reflFields[$idField]->getValue($entity);
+                $value = $this->getIndentifierValue($entity, $idField);
 
                 if (null !== $value) {
                     $id[$idField] = $value;
@@ -729,7 +730,7 @@ class ClassMetadataInfo implements ClassMetadata
         }
 
         $id = $this->identifier[0];
-        $value = $this->reflFields[$id]->getValue($entity);
+        $value = $this->getIndentifierValue($entity, $id);
 
         if (null === $value) {
             return [];
@@ -738,6 +739,15 @@ class ClassMetadataInfo implements ClassMetadata
         return [$id => $value];
     }
 
+    private function getIndentifierValue($entity, $id)
+    {
+        if ($entity instanceof AssociationCacheEntry) {
+             return $entity->identifier[$id];
+        }
+
+        return $this->reflFields[$id]->getValue($entity);
+    }
+
     /**
      * Populates the entity identifier of an entity.
      *
diff --git a/tests/Doctrine/Tests/ORM/Functional/Ticket/GH6217Test.php b/tests/Doctrine/Tests/ORM/Functional/Ticket/GH6217Test.php
new file mode 100644
index 000000000..d87b9642e
--- /dev/null
+++ b/tests/Doctrine/Tests/ORM/Functional/Ticket/GH6217Test.php
@@ -0,0 +1,162 @@
+<?php
+namespace Doctrine\Tests\Functional\Ticket;
+
+use Doctrine\Tests\OrmFunctionalTestCase;
+
+final class GH6217Test extends OrmFunctionalTestCase
+{
+    public function setUp()
+    {
+        $this->enableSecondLevelCache();
+
+        parent::setUp();
+
+        $this->_schemaTool->createSchema(
+            [
+                $this->_em->getClassMetadata(GH6217User::class),
+                $this->_em->getClassMetadata(GH6217Profile::class),
+                $this->_em->getClassMetadata(GH6217Category::class),
+                $this->_em->getClassMetadata(GH6217UserProfile::class),
+            ]
+        );
+    }
+
+    /**
+     * @group 6217
+     */
+    public function testRetrievingCacheShouldNotThrowUndefinedIndexException()
+    {
+        $user = new GH6217User(1, 'user 1');
+        $profile = new GH6217Profile(1);
+        $category = new GH6217Category(1, 'category 1');
+        $userProfile = new GH6217UserProfile($user, $profile, $category);
+
+        $this->_em->persist($category);
+        $this->_em->persist($user);
+        $this->_em->persist($profile);
+        $this->_em->persist($userProfile);
+        $this->_em->flush();
+        $this->_em->clear();
+
+        $repository = $this->_em->getRepository(GH6217UserProfile::class);
+        $filters = ['user' => 1, 'category' => 1];
+
+        $this->assertCount(1, $repository->findBy($filters));
+        $queryCount = $this->getCurrentQueryCount();
+
+        $this->_em->clear();
+
+        $this->assertCount(1, $repository->findBy($filters));
+        $this->assertEquals($queryCount, $this->getCurrentQueryCount());
+    }
+}
+
+/**
+ * @Entity
+ * @Cache(usage="NONSTRICT_READ_WRITE")
+ */
+class GH6217User
+{
+    /**
+     * @Id
+     * @Column(type="integer")
+     *
+     * @var int
+     */
+    public $id;
+
+    /**
+     * @Column(type="string", length=60, unique=true)
+     *
+     * @var string
+     */
+    public $username;
+
+    public function __construct(int $id, string $username)
+    {
+        $this->id = $id;
+        $this->username = $username;
+    }
+}
+
+/**
+ * @Entity
+ * @Cache(usage="NONSTRICT_READ_WRITE")
+ */
+class GH6217Profile
+{
+    /**
+     * @Id
+     * @Column(type="integer")
+     *
+     * @var int
+     */
+    public $id;
+
+    public function __construct(int $id)
+    {
+        $this->id = $id;
+    }
+}
+
+/**
+ * @Entity
+ * @Cache(usage="NONSTRICT_READ_WRITE")
+ */
+class GH6217Category
+{
+    /**
+     * @Id
+     * @Column(type="integer")
+     *
+     * @var int
+     */
+    public $id;
+
+    public function __construct(int $id)
+    {
+        $this->id = $id;
+    }
+}
+
+/**
+ * @Entity
+ * @Cache(usage="NONSTRICT_READ_WRITE")
+ */
+class GH6217UserProfile
+{
+    /**
+     * @Id
+     * @Cache("NONSTRICT_READ_WRITE")
+     * @ManyToOne(targetEntity="GH6217User")
+     * @JoinColumn(nullable=false)
+     *
+     * @var GH6217User
+     */
+    public $user;
+
+    /**
+     * @Id
+     * @Cache("NONSTRICT_READ_WRITE")
+     * @ManyToOne(targetEntity="GH6217Profile", fetch="EAGER")
+     *
+     * @var GH6217Profile
+     */
+    public $profile;
+
+    /**
+     * @Id
+     * @Cache("NONSTRICT_READ_WRITE")
+     * @ManyToOne(targetEntity="GH6217Category", fetch="EAGER")
+     *
+     * @var GH6217Category
+     */
+    public $category;
+
+    public function __construct(GH6217User $user, GH6217Profile $profile, GH6217Category $category)
+    {
+        $this->user = $user;
+        $this->profile = $profile;
+        $this->category = $category;
+    }
+}