diff --git a/app/dao/organisation_dao.py b/app/dao/organisation_dao.py
index bdd430c8c..26b6dca1f 100644
--- a/app/dao/organisation_dao.py
+++ b/app/dao/organisation_dao.py
@@ -59,12 +59,12 @@ def dao_update_organisation(organisation_id, **kwargs):
         kwargs
     )
 
-    if domains:
+    if isinstance(domains, list):
 
         Domain.query.filter_by(organisation_id=organisation_id).delete()
 
         db.session.bulk_save_objects([
-            Domain(domain=domain, organisation_id=organisation_id)
+            Domain(domain=domain.lower(), organisation_id=organisation_id)
             for domain in domains
         ])
 
diff --git a/tests/app/dao/test_organisation_dao.py b/tests/app/dao/test_organisation_dao.py
index 6375cf7b6..667fc1438 100644
--- a/tests/app/dao/test_organisation_dao.py
+++ b/tests/app/dao/test_organisation_dao.py
@@ -90,6 +90,35 @@ def test_update_organisation(
         assert getattr(organisation, attribute) == value
 
 
+@pytest.mark.parametrize('domain_list, expected_domains', (
+    (['abc', 'def'], {'abc', 'def'}),
+    (['ABC', 'DEF'], {'abc', 'def'}),
+    ([], set()),
+    (None, {'123', '456'}),
+    pytest.param(
+        ['abc', 'ABC'], {'abc'},
+        marks=pytest.mark.xfail(raises=IntegrityError)
+    ),
+))
+def test_update_organisation_domains_lowercases(
+    notify_db,
+    notify_db_session,
+    domain_list,
+    expected_domains,
+):
+    create_organisation()
+
+    organisation = Organisation.query.one()
+
+    # Seed some domains
+    dao_update_organisation(organisation.id, domains=['123', '456'])
+
+    # This should overwrite the seeded domains
+    dao_update_organisation(organisation.id, domains=domain_list)
+
+    assert {domain.domain for domain in organisation.domains} == expected_domains
+
+
 def test_add_service_to_organisation(notify_db, notify_db_session, sample_service, sample_organisation):
     assert sample_organisation.services == []