diff --git a/tests/regressiontests/serializers_regress/models.py b/tests/regressiontests/serializers_regress/models.py
index 268fa052e6..f8bcb34d06 100644
--- a/tests/regressiontests/serializers_regress/models.py
+++ b/tests/regressiontests/serializers_regress/models.py
@@ -117,7 +117,8 @@ class M2MData(models.Model):
     data = models.ManyToManyField(Anchor, null=True)
     
 class O2OData(models.Model):
-    data = models.OneToOneField(Anchor, null=True)
+    # One to one field can't be null, since it is a PK.
+    data = models.OneToOneField(Anchor)
 
 class FKSelfData(models.Model):
     data = models.ForeignKey('self', null=True)