diff --git a/themis/fields/__init__.py b/themis/fields/__init__.py
index 90f29e8..2a272ea 100644
--- a/themis/fields/__init__.py
+++ b/themis/fields/__init__.py
@@ -1,5 +1,5 @@
 from zope.interface import implements, implementsOnly
-from zope.schema import Choice, Field, List, Date, Orderable
+from zope.schema import Choice, Field, List, Date, Orderable, TextLine
 
 from zope.schema.interfaces import IFromUnicode, IChoice
 from zope.schema.interfaces import WrongType
@@ -354,7 +354,6 @@ class RelatedDoc(Relation):
         return clone
 
 
-class MailRefId(Field):
-    implements(IMailRefId)
-
+class MailRefId(TextLine):
+    implements(IMailRefId, IFromUnicode)