diff --git a/nova/compute/api.py b/nova/compute/api.py
index 336601fcaadc..f898acda802c 100644
--- a/nova/compute/api.py
+++ b/nova/compute/api.py
@@ -1310,7 +1310,7 @@ class API(base.Base):
                     "(source: 'blank', dest: 'volume') need to have non-zero "
                     "size"))
 
-        ephemeral_size = sum(bdm.volume_size or 0
+        ephemeral_size = sum(bdm.volume_size or instance_type['ephemeral_gb']
                 for bdm in block_device_mappings
                 if block_device.new_format_is_ephemeral(bdm))
         if ephemeral_size > instance_type['ephemeral_gb']:
diff --git a/nova/exception.py b/nova/exception.py
index 234d502b9f6a..6fbed08ef84a 100644
--- a/nova/exception.py
+++ b/nova/exception.py
@@ -219,7 +219,9 @@ class InvalidBDMLocalsLimit(InvalidBDM):
 
 class InvalidBDMEphemeralSize(InvalidBDM):
     msg_fmt = _("Ephemeral disks requested are larger than "
-                "the instance type allows.")
+                "the instance type allows. If no size is given "
+                "in one block device mapping, flavor ephemeral "
+                "size will be used.")
 
 
 class InvalidBDMSwapSize(InvalidBDM):
diff --git a/nova/tests/unit/compute/test_compute.py b/nova/tests/unit/compute/test_compute.py
index 643491902035..6b6b7101026c 100644
--- a/nova/tests/unit/compute/test_compute.py
+++ b/nova/tests/unit/compute/test_compute.py
@@ -1056,6 +1056,40 @@ class ComputeVolumeTestCase(BaseTestCase):
                           self.context, instance, instance_type,
                           mappings_)
 
+    def test_validate_bdm_with_more_than_one_default(self):
+        instance_type = {'swap': 1, 'ephemeral_gb': 1}
+        all_mappings = [fake_block_device.FakeDbBlockDeviceDict({
+                         'id': 1,
+                         'no_device': None,
+                         'source_type': 'volume',
+                         'destination_type': 'volume',
+                         'snapshot_id': None,
+                         'volume_size': 1,
+                         'device_name': 'vda',
+                         'boot_index': 0,
+                         'delete_on_termination': False}, anon=True),
+                        fake_block_device.FakeDbBlockDeviceDict({
+                         'device_name': '/dev/vdb',
+                         'source_type': 'blank',
+                         'destination_type': 'local',
+                         'device_type': 'disk',
+                         'volume_size': None,
+                         'boot_index': -1}, anon=True),
+                        fake_block_device.FakeDbBlockDeviceDict({
+                         'device_name': '/dev/vdc',
+                         'source_type': 'blank',
+                         'destination_type': 'local',
+                         'device_type': 'disk',
+                         'volume_size': None,
+                         'boot_index': -1}, anon=True)]
+        all_mappings = block_device_obj.block_device_make_list_from_dicts(
+                self.context, all_mappings)
+
+        self.assertRaises(exception.InvalidBDMEphemeralSize,
+                          self.compute_api._validate_bdm,
+                          self.context, self.instance,
+                          instance_type, all_mappings)
+
     def test_validate_bdm_media_service_exceptions(self):
         instance_type = {'swap': 1, 'ephemeral_gb': 1}
         bdms = [fake_block_device.FakeDbBlockDeviceDict({