diff --git a/scrapy/contrib/loader/__init__.py b/scrapy/contrib/loader/__init__.py
index 7be2524b9..c861f8a24 100644
--- a/scrapy/contrib/loader/__init__.py
+++ b/scrapy/contrib/loader/__init__.py
@@ -80,7 +80,7 @@ class ItemLoader(object):
 
     def load_item(self):
         item = self.item
-        for field_name in self._values:
+        for field_name in tuple(self._values):
             value = self.get_output_value(field_name)
             if value is not None:
                 item[field_name] = value