diff --git a/openerp/osv/orm.py b/openerp/osv/orm.py
index 48b607273de..625247d464f 100644
--- a/openerp/osv/orm.py
+++ b/openerp/osv/orm.py
@@ -1794,11 +1794,12 @@ class orm_template(object):
         fields_to_search = set(
             field for field, descriptor in fields.iteritems()
             if descriptor.get('select'))
+
         for view in (form_view, tree_view):
             view_root = etree.fromstring(view['arch'])
             # Only care about select=1 in xpath below, because select=2 is covered
             # by the custom advanced search in clients
-            fields_to_search = fields_to_search.union(view_root.xpath("//field[@select=1]/@name"))
+            fields_to_search.update(view_root.xpath("//field[@select=1]/@name"))
 
         tree_view_root = view_root # as provided by loop above
         search_view = etree.Element("search", attrib={'string': tree_view_root.get("string", "")})