diff --git a/src/Indexer.php b/src/Indexer.php
index ddb1166..fd4681f 100644
--- a/src/Indexer.php
+++ b/src/Indexer.php
@@ -18,13 +18,18 @@ class Indexer
 
     public function __construct(Client $client, SerializerInterface $serializer, int $bulkMaxSize = 100)
     {
-        // TODO: on the destruct, maybe throw an exception for non empty indexer queues?
-
         $this->client = $client;
         $this->bulkMaxSize = $bulkMaxSize ?? 100;
         $this->serializer = $serializer;
     }
 
+    public function __destruct()
+    {
+        if ($this->getQueueSize() > 0) {
+            throw new \RuntimeException(sprintf('%s queue is not empty, you need to call flush() to send documents to Elasticsearch or clear() to empty the queue.', __CLASS__));
+        }
+    }
+
     public function scheduleIndex($index, Document $document)
     {
         $document->setIndex($index instanceof Index ? $index->getName() : $index);
@@ -104,6 +109,11 @@ public function flush(): ?Bulk\ResponseSet
         return $response;
     }
 
+    public function clear(): void
+    {
+        $this->currentBulk = null;
+    }
+
     public function getQueueSize()
     {
         if (null === $this->currentBulk) {
diff --git a/tests/IndexerTest.php b/tests/IndexerTest.php
index 677cc98..92527fa 100644
--- a/tests/IndexerTest.php
+++ b/tests/IndexerTest.php
@@ -39,6 +39,30 @@ public function testIndexOneDocument(): void
         $this->assertEquals('f', $document->getId());
     }
 
+    public function testIndexDocumentsWithoutFlushingThrowError(): void
+    {
+        $indexName = mb_strtolower(__FUNCTION__);
+        $dto = new TestDTO();
+        $dto->bar = 'Oops I forgot the flush.';
+
+        $indexer = $this->getIndexer();
+
+        $indexer->scheduleIndex($indexName, new Document('f', $dto));
+        $this->assertEquals(1, $indexer->getQueueSize());
+        $indexer->clear();
+        $this->assertEquals(0, $indexer->getQueueSize());
+
+        $indexer->scheduleIndex($indexName, new Document('f', $dto));
+        try {
+            $indexer->__destruct();
+            $this->assertFalse(true, 'This code should not be accessible.');
+        } catch (\RuntimeException $exception) {
+            $this->assertStringContainsString('queue is not empty', $exception->getMessage());
+        }
+
+        $indexer->clear(); // avoid fatal error when the test ends
+    }
+
     public function testIndexOneDocumentWithMapping(): void
     {
         $indexName = mb_strtolower(__FUNCTION__);