diff --git a/scrapy/core/engine.py b/scrapy/core/engine.py
index 37fe0a873..b83f0fc48 100644
--- a/scrapy/core/engine.py
+++ b/scrapy/core/engine.py
@@ -118,21 +118,23 @@ class ExecutionEngine(object):
         if self.paused:
             return
 
-        while not self._needs_backout(spider):
-            if not self._next_request_from_scheduler(spider):
-                break
-
-        if slot.start_requests and not self._needs_backout(spider):
+        while not self._needs_backout(spider) and slot.start_requests:
             try:
                 request = next(slot.start_requests)
+                if request is None:
+                    break
+                self.crawl(request, spider)
+
             except StopIteration:
                 slot.start_requests = None
             except Exception:
                 slot.start_requests = None
                 logger.error('Error while obtaining start requests',
                              exc_info=True, extra={'spider': spider})
-            else:
-                self.crawl(request, spider)
+
+        while not self._needs_backout(spider):
+            if not self._next_request_from_scheduler(spider):
+                break
 
         if self.spider_is_idle(spider) and slot.close_if_idle:
             self._spider_idle(spider)