diff --git a/sorteerhoed/HITStore.py b/sorteerhoed/HITStore.py
index be1ba52..f557fb0 100644
--- a/sorteerhoed/HITStore.py
+++ b/sorteerhoed/HITStore.py
@@ -134,6 +134,7 @@ class HIT(Base):
         session = object_session(self)
         q = session.query(Assignment).\
             filter(Assignment.submit_page_at < self.created_at).\
+            filter(Assignment.worker_id != None).\
             group_by(Assignment.worker_id).\
             order_by(Assignment.created_at.desc())
         if assignment and assignment.worker_id: