diff --git a/cern_access/indico_cern_access/migrations/20241015_1702_92377810f14e_make_reservation_code_unique_and_nullable.py b/cern_access/indico_cern_access/migrations/20241015_1702_92377810f14e_make_reservation_code_unique_and_nullable.py
new file mode 100644
index 00000000..6a863579
--- /dev/null
+++ b/cern_access/indico_cern_access/migrations/20241015_1702_92377810f14e_make_reservation_code_unique_and_nullable.py
@@ -0,0 +1,27 @@
+"""Make reservation code unique and nullable
+
+Revision ID: 92377810f14e
+Revises: ee88b64f9494
+Create Date: 2024-10-15 17:02:59.886902
+"""
+
+from alembic import op
+
+
+# revision identifiers, used by Alembic.
+revision = '92377810f14e'
+down_revision = 'ee88b64f9494'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.alter_column('access_requests', 'reservation_code', nullable=True, schema='plugin_cern_access')
+    op.execute("UPDATE plugin_cern_access.access_requests SET reservation_code = NULL WHERE reservation_code = ''")
+    op.create_unique_constraint(None, 'access_requests', ['reservation_code'], schema='plugin_cern_access')
+
+
+def downgrade():
+    op.drop_constraint('uq_access_requests_reservation_code', 'access_requests', schema='plugin_cern_access')
+    op.execute("UPDATE plugin_cern_access.access_requests SET reservation_code = '' WHERE reservation_code IS NULL")
+    op.alter_column('access_requests', 'reservation_code', nullable=False, schema='plugin_cern_access')
diff --git a/cern_access/indico_cern_access/models/access_requests.py b/cern_access/indico_cern_access/models/access_requests.py
index 65eeb310..cffc63a4 100644
--- a/cern_access/indico_cern_access/models/access_requests.py
+++ b/cern_access/indico_cern_access/models/access_requests.py
@@ -39,7 +39,8 @@ class CERNAccessRequest(db.Model):
     )
     reservation_code = db.Column(
         db.String,
-        nullable=False
+        nullable=True,
+        unique=True
     )
     adams_nonce = db.Column(
         db.String,
diff --git a/cern_access/indico_cern_access/plugin.py b/cern_access/indico_cern_access/plugin.py
index 01864f7c..71f1cc05 100644
--- a/cern_access/indico_cern_access/plugin.py
+++ b/cern_access/indico_cern_access/plugin.py
@@ -272,7 +272,6 @@ def _registration_created(self, registration, management, **kwargs):
             license_plate=access_request_data['license_plate'],
             accompanying_persons=sanitize_accompanying_persons(accompanying_persons, registration),
             request_state=CERNAccessRequestState.not_requested,
-            reservation_code=''
         )
 
     def _event_time_changed(self, sender, obj, **kwargs):
diff --git a/cern_access/indico_cern_access/util.py b/cern_access/indico_cern_access/util.py
index ed28a0d9..9779978e 100644
--- a/cern_access/indico_cern_access/util.py
+++ b/cern_access/indico_cern_access/util.py
@@ -269,7 +269,11 @@ def withdraw_event_access_request(req):
 def get_random_reservation_code():
     """Generate random reservation code for data required by ADaMS API."""
     charset = 'ABCDEFGHIJKLMNPQRSTUVWXYZ123456789'
-    return 'I' + ''.join(random.sample(charset, 6))
+
+    while True:
+        reservation_code = 'I' + ''.join(random.sample(charset, 6))
+        if not CERNAccessRequest.query.filter_by(reservation_code=reservation_code).has_rows():
+            return reservation_code
 
 
 def create_access_request(registration, state, reservation_code, nonce):
diff --git a/cern_access/tests/conftest.py b/cern_access/tests/conftest.py
index 288bf060..24e43276 100644
--- a/cern_access/tests/conftest.py
+++ b/cern_access/tests/conftest.py
@@ -159,5 +159,4 @@ def dummy_access_request(dummy_regform, accompanying_persons_field):
                                                 birth_place='bar',
                                                 license_plate=None,
                                                 request_state=CERNAccessRequestState.not_requested,
-                                                reservation_code='',
                                                 accompanying_persons=accompaning_persons)