diff --git a/themis/fields/__init__.py b/themis/fields/__init__.py
index 623160e..219b2dd 100644
--- a/themis/fields/__init__.py
+++ b/themis/fields/__init__.py
@@ -36,8 +36,8 @@ from themis.fields.interfaces import IDocHistoLine, IDocHistoLines
 
 from themis.fields.vocabs import SubjectsSource
 from themis.fields.vocabs import LegislativeSessionsSource
+from themis.fields.vocabs import RelatedDocObjPathSourceBinder
 
-from plone.formwidget.contenttree import ObjPathSourceBinder
 from z3c.relationfield.schema import Relation, RelationList
 
 
@@ -349,14 +349,14 @@ class RelatedDoc(Relation):
 
     def __init__(self, **kw):
         super(RelatedDoc, self).__init__(**kw)
-        self.vocabulary = ObjPathSourceBinder()
+        self.vocabulary = RelatedDocObjPathSourceBinder()
 
     source = property(lambda self: self.vocabulary)
 
     def bind(self, object):
         """See zope.schema._bootstrapinterfaces.IField."""
         clone = super(RelatedDoc, self).bind(object)
-        clone.vocabulary = self.vocabulary(object)
+        clone.vocabulary = RelatedDocObjPathSourceBinder()(object)
         return clone
 
 
diff --git a/themis/fields/vocabs.py b/themis/fields/vocabs.py
index 91e3a18..1cff646 100644
--- a/themis/fields/vocabs.py
+++ b/themis/fields/vocabs.py
@@ -13,6 +13,7 @@ from zope.schema.vocabulary import SimpleVocabulary, SimpleTerm
 from Products.CMFCore.utils import getToolByName
 
 from plone.registry.interfaces import IRegistry
+from plone.formwidget.contenttree import ObjPathSourceBinder
 
 try:
     from tabellio.config.interfaces import ITabellioSettings
@@ -266,3 +267,8 @@ class LegislativeSessionsSource(object): # Legislative sessions
     def __iter__(self):
         # hack to let schema editor handle the field
         yield u'DO NOT TOUCH'
+
+class RelatedDocObjPathSourceBinder(ObjPathSourceBinder):
+    def __iter__(self):
+        # hack to let schema editor handle the field
+        yield u'DO NOT TOUCH'