diff --git a/gensim/models/keyedvectors.py b/gensim/models/keyedvectors.py
index daa5482184..2fb0e15b20 100644
--- a/gensim/models/keyedvectors.py
+++ b/gensim/models/keyedvectors.py
@@ -258,6 +258,9 @@ def _load_specials(self, *args, **kwargs):
         # fixup rename of vocab into map
         if 'key_to_index' not in self.__dict__:
             self._upconvert_old_vocab()
+        # ensure older instances have next_index
+        if not hasattr(self, 'next_index'):
+            self.next_index = len(self)
 
     def _upconvert_old_vocab(self):
         """Convert a loaded, pre-gensim-4.0.0 version instance that had a 'vocab' dict of data objects."""