diff --git a/themis/importexport/sync.py b/themis/importexport/sync.py
index 70dfaf9..fadc7d7 100644
--- a/themis/importexport/sync.py
+++ b/themis/importexport/sync.py
@@ -341,6 +341,8 @@ class SyncFromThemis(UtilityView):
                 object.author_is_government = True
             elif authors_v:
                 authors_intids = [self.get_author_intid(x) for x in authors_v]
+                if None in authors_intids:
+                    log.warn('unknown error in doc %s' % object.id)
                 object.authors = [RelationValue(x) for x in authors_intids if x]
                 object.author_is_government = False
             else: