2 files changed, 3 insertions, 3 deletions
diff --git a/slixmpp/plugins/xep_0004/stanza/field.py b/slixmpp/plugins/xep_0004/stanza/field.py
index 42f1210b..2bcc823a 100644
--- a/slixmpp/plugins/xep_0004/stanza/field.py
+++ b/slixmpp/plugins/xep_0004/stanza/field.py
@@ -16,7 +16,7 @@ class FormField(ElementBase):
     plugin_multi_attrib = 'fields'
     interfaces = set(('answer', 'desc', 'required', 'value',
                       'label', 'type', 'var'))
-    sub_interfaces = set(('desc',))
+    sub_interfaces = {'desc'}
     plugin_tag_map = {}
     plugin_attrib_map = {}
 
@@ -165,7 +165,7 @@ class FieldOption(ElementBase):
     name = 'option'
     plugin_attrib = 'option'
     interfaces = set(('label', 'value'))
-    sub_interfaces = set(('value',))
+    sub_interfaces = {'value'}
     plugin_multi_attrib = 'options'
 
 
diff --git a/slixmpp/plugins/xep_0004/stanza/form.py b/slixmpp/plugins/xep_0004/stanza/form.py
index 32bfc0e3..52b18f81 100644
--- a/slixmpp/plugins/xep_0004/stanza/form.py
+++ b/slixmpp/plugins/xep_0004/stanza/form.py
@@ -24,7 +24,7 @@ class Form(ElementBase):
     name = 'x'
     plugin_attrib = 'form'
     interfaces = OrderedSet(('instructions', 'reported', 'title', 'type', 'items', ))
-    sub_interfaces = set(('title',))
+    sub_interfaces = {'title'}
     form_types = set(('cancel', 'form', 'result', 'submit'))
 
     def __init__(self, *args, **kwargs):