diff --git a/django/db/models/base.py b/django/db/models/base.py
index 8b021978ed..064dc8fdec 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -209,7 +209,7 @@ class Model(object):
 
         # First, try an UPDATE. If that doesn't update anything, do an INSERT.
         pk_val = self._get_pk_val()
-        pk_set = bool(pk_val)
+        pk_set = pk_val is not None
         record_exists = True
         if pk_set:
             # Determine whether a record with the primary key already exists.