diff --git a/scrapy/core/engine.py b/scrapy/core/engine.py
index 004a65b98..6f1352d6a 100644
--- a/scrapy/core/engine.py
+++ b/scrapy/core/engine.py
@@ -228,6 +228,9 @@ class ExecutionEngine(object):
             self.next_request(spider)
             return _
 
+        if spider not in self.downloader.sites:
+            return defer.fail(Failure(IgnoreRequest())).addBoth(_on_complete)
+
         dwld = mustbe_deferred(self.downloader.fetch, request, spider)
         dwld.addCallbacks(_on_success, _on_error)
         dwld.addBoth(_on_complete)