diff --git a/scrapy/contrib/web/service.py b/scrapy/contrib/web/service.py
index 8db4cc1ac..b2b51bc37 100644
--- a/scrapy/contrib/web/service.py
+++ b/scrapy/contrib/web/service.py
@@ -9,7 +9,7 @@ from scrapy.http import Request
 from scrapy.item import ScrapedItem
 from scrapy.core.exceptions import NotConfigured
 from scrapy.conf import settings
-from scrapy.utils.misc import memoize
+from scrapy.utils.misc import memoize, arg_to_iter
 from lrucache import LRUCache
 
 from .site import WebSite, WebResource
@@ -63,7 +63,7 @@ def url_to_guid(httprequest):
         except Exception, ex:
             return _response(message='Error processing url')
 
-        guids = [i.guid for i in items if isinstance(i, ScrapedItem)]
+        guids = [i.guid for i in arg_to_iter(items) if isinstance(i, ScrapedItem)]
         return _response(guids=guids)
 
     deferred = defer.Deferred().addCallbacks(_on_success, _on_error)