diff --git a/pagure/lib/model.py b/pagure/lib/model.py
index 9b8e5ec..dd58bef 100644
--- a/pagure/lib/model.py
+++ b/pagure/lib/model.py
@@ -701,7 +701,7 @@ class PullRequest(BASE):
     closed_by_id = sa.Column(
         sa.Integer,
         sa.ForeignKey('users.id', onupdate='CASCADE'),
-        nullable=False)
+        nullable=True)
 
     date_created = sa.Column(sa.DateTime, nullable=False,
                              default=datetime.datetime.utcnow)