diff --git a/django/forms/models.py b/django/forms/models.py
index af3d7023c8..78788fc2db 100644
--- a/django/forms/models.py
+++ b/django/forms/models.py
@@ -676,7 +676,7 @@ class BaseModelFormSet(BaseFormSet):
                 pk_value = form.instance.pk
             else:
                 try:
-                    if index:
+                    if index is not None:
                         pk_value = self.get_queryset()[index].pk
                     else:
                         pk_value = None