diff --git a/scrapy/crawler.py b/scrapy/crawler.py
index bdcfa9d0c..ef99c243a 100644
--- a/scrapy/crawler.py
+++ b/scrapy/crawler.py
@@ -189,7 +189,7 @@ class CrawlerRunner(object):
 
         Returns a deferred that is fired when they all have ended.
         """
-        return defer.DeferredList([c.stop() for c in self.crawlers])
+        return defer.DeferredList([c.stop() for c in list(self.crawlers)])
 
     @defer.inlineCallbacks
     def join(self):