diff --git a/lib/Doctrine/ORM/UnitOfWork.php b/lib/Doctrine/ORM/UnitOfWork.php
index 498c4a699..b45e4921b 100644
--- a/lib/Doctrine/ORM/UnitOfWork.php
+++ b/lib/Doctrine/ORM/UnitOfWork.php
@@ -2270,6 +2270,10 @@ class UnitOfWork implements PropertyChangedListener
      */
     public function lock($entity, $lockMode, $lockVersion = null)
     {
+        if ($entity === null) {
+            throw new \InvalidArgumentException("No entity passed to UnitOfWork#lock().");
+        }
+
         if ($this->getEntityState($entity, self::STATE_DETACHED) != self::STATE_MANAGED) {
             throw ORMInvalidArgumentException::entityNotManaged($entity);
         }
diff --git a/tests/Doctrine/Tests/ORM/UnitOfWorkTest.php b/tests/Doctrine/Tests/ORM/UnitOfWorkTest.php
index 9a645d52e..97f60f859 100644
--- a/tests/Doctrine/Tests/ORM/UnitOfWorkTest.php
+++ b/tests/Doctrine/Tests/ORM/UnitOfWorkTest.php
@@ -220,6 +220,15 @@ class UnitOfWorkTest extends \Doctrine\Tests\OrmTestCase
         // This commit should not raise an E_NOTICE
         $this->_unitOfWork->commit();
     }
+
+    /**
+     * @group DDC-1984
+     */
+    public function testLockWithoutEntityThrowsException()
+    {
+        $this->setExpectedException('InvalidArgumentException');
+        $this->_unitOfWork->lock(null, null, null);
+    }
 }
 
 /**