diff --git a/lib/Doctrine/ORM/UnitOfWork.php b/lib/Doctrine/ORM/UnitOfWork.php
index c0b0dd8c302..56615807f4e 100644
--- a/lib/Doctrine/ORM/UnitOfWork.php
+++ b/lib/Doctrine/ORM/UnitOfWork.php
@@ -1154,7 +1154,11 @@ private function getCommitOrder(array $entityChangeSet = null)
 
                 $joinColumns = reset($assoc['joinColumns']);
 
-                $calc->addDependency($targetClass->name, $class->name, (int)empty($joinColumns['nullable']));
+                $calc->addDependency(
+                    $targetClass->name,
+                    $class->name,
+                    (int) (($joinColumns['nullable'] ?? true) === false)
+                );
 
                 // If the target class has mapped subclasses, these share the same dependency.
                 if ( ! $targetClass->subClasses) {
diff --git a/tests/Doctrine/Tests/ORM/Functional/Ticket/DDC6499Test.php b/tests/Doctrine/Tests/ORM/Functional/Ticket/DDC6499Test.php
new file mode 100644
index 00000000000..0d86fffb037
--- /dev/null
+++ b/tests/Doctrine/Tests/ORM/Functional/Ticket/DDC6499Test.php
@@ -0,0 +1,96 @@
+<?php
+
+namespace Doctrine\Tests\Functional\Ticket;
+
+use Doctrine\Common\Collections\ArrayCollection;
+use Doctrine\Tests\OrmFunctionalTestCase;
+
+/**
+ * @group #6499
+ *
+ *
+ * Specifically, DDC6499B has a dependency on DDC6499A, and DDC6499A
+ * has a dependency on DDC6499B. Since DDC6499A#b is not nullable,
+ * the DDC6499B should be inserted first.
+ */
+class DDC6499Test extends OrmFunctionalTestCase
+{
+    /**
+     * {@inheritDoc}
+     */
+    protected function setUp() : void
+    {
+        parent::setUp();
+
+        $this->_schemaTool->createSchema([
+            $this->_em->getClassMetadata(DDC6499A::class),
+            $this->_em->getClassMetadata(DDC6499B::class),
+        ]);
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    protected function tearDown() : void
+    {
+        parent::tearDown();
+
+        $this->_schemaTool->dropSchema([
+            $this->_em->getClassMetadata(DDC6499A::class),
+            $this->_em->getClassMetadata(DDC6499B::class),
+        ]);
+    }
+
+    public function testIssue() : void
+    {
+        $b = new DDC6499B();
+        $a = new DDC6499A();
+
+        $this->_em->persist($a);
+
+        $a->b = $b;
+
+        $this->_em->persist($b);
+
+        $this->_em->flush();
+
+        self::assertInternalType('integer', $a->id);
+        self::assertInternalType('integer', $b->id);
+    }
+
+    public function testIssueReversed() : void
+    {
+        $b = new DDC6499B();
+        $a = new DDC6499A();
+
+        $a->b = $b;
+
+        $this->_em->persist($b);
+        $this->_em->persist($a);
+
+        $this->_em->flush();
+
+        self::assertInternalType('integer', $a->id);
+        self::assertInternalType('integer', $b->id);
+    }
+}
+
+/** @Entity */
+class DDC6499A
+{
+    /** @Id @Column(type="integer") @GeneratedValue */
+    public $id;
+
+    /** @JoinColumn(nullable=false) @OneToOne(targetEntity=DDC6499B::class) */
+    public $b;
+}
+
+/** @Entity */
+class DDC6499B
+{
+    /** @Id @Column(type="integer") @GeneratedValue */
+    public $id;
+
+    /** @ManyToOne(targetEntity="DDC6499A") */
+    private $a;
+}