diff --git a/themis/fields/__init__.py b/themis/fields/__init__.py
index 6c2b4fe..ef012c9 100644
--- a/themis/fields/__init__.py
+++ b/themis/fields/__init__.py
@@ -420,6 +420,8 @@ class DocHistoLines(Field):
 class PreviewDoc(Field):
     implements(IPreviewDoc, IFromUnicode)
 
+    attribute_name = None
+
     def __init__(self, **kw):
         for attr in ('height', 'width', 'attribute_name'):
             if kw.get(attr):