diff --git a/neutron_lib/api/definitions/subnet_onboard.py b/neutron_lib/api/definitions/subnet_onboard.py
index a57feef35..30efa5743 100644
--- a/neutron_lib/api/definitions/subnet_onboard.py
+++ b/neutron_lib/api/definitions/subnet_onboard.py
@@ -41,7 +41,7 @@ ONBOARD_SUBNETS_SPECS = {
 RESOURCE_ATTRIBUTE_MAP = {
     subnetpool_def.COLLECTION_NAME: {
         subnet_def.COLLECTION_NAME: {
-            'allow_post': True, 'allow_put': True,
+            'allow_post': True, 'allow_put': True, 'default': [],
             'validate': {
                 'type:list_of_any_key_specs_or_none': ONBOARD_SUBNETS_SPECS
             }