diff --git a/api/models/account.py b/api/models/account.py
index acd2441428..9436d9553d 100644
--- a/api/models/account.py
+++ b/api/models/account.py
@@ -272,7 +272,7 @@ class InvitationCode(Base):
     batch: Mapped[str] = mapped_column(db.String(255))
     code: Mapped[str] = mapped_column(db.String(32))
     status: Mapped[str] = mapped_column(db.String(16), server_default=db.text("'unused'::character varying"))
-    used_at: Mapped[datetime] = mapped_column(db.DateTime)
+    used_at: Mapped[Optional[datetime]] = mapped_column(db.DateTime)
     used_by_tenant_id: Mapped[Optional[str]] = mapped_column(StringUUID)
     used_by_account_id: Mapped[Optional[str]] = mapped_column(StringUUID)
     deprecated_at: Mapped[Optional[datetime]] = mapped_column(db.DateTime, nullable=True)