diff --git a/ckanext/harvest/helpers.py b/ckanext/harvest/helpers.py
index 6d969f9d..1bc4a230 100644
--- a/ckanext/harvest/helpers.py
+++ b/ckanext/harvest/helpers.py
@@ -133,7 +133,7 @@ def harvest_source_extra_fields():
     for harvester in p.PluginImplementations(IHarvester):
         if not hasattr(harvester, 'extra_schema'):
             continue
-        fields[harvester.info()['name']] = harvester.extra_schema().keys()
+        fields[harvester.info()['name']] = list(harvester.extra_schema().keys())
     return fields