diff --git a/tabellio/documents/document.py b/tabellio/documents/document.py
index 6f991bc..4bc99ec 100644
--- a/tabellio/documents/document.py
+++ b/tabellio/documents/document.py
@@ -92,7 +92,7 @@ class Document(Item, BasePublication):
     def related_elements(self, max_items=5, closeness=0.5):
         r = BasePublication.related_elements(self, max_items, closeness)
         doc_ids = [x.id for x in self.associated_docs]
-        return [x for x in r if x.getId() not in doc_ids]
+        return [x for x in r if ((type(x.getId) is str) and x.getId or x.getId()) not in doc_ids]
 
     @property
     def publication_date_str(self):