diff --git a/neutron_lib/tests/unit/utils/test_net.py b/neutron_lib/tests/unit/utils/test_net.py
index 0d88e57e9..0a8c36721 100644
--- a/neutron_lib/tests/unit/utils/test_net.py
+++ b/neutron_lib/tests/unit/utils/test_net.py
@@ -77,6 +77,14 @@ class TestRandomMacGenerator(base.BaseTestCase):
         self.assertEqual(['aa:bb:00:a2:a2:a2'], list(generator))
         mock_rnd.assert_called_with(8)
 
+    @mock.patch.object(random, 'getrandbits', return_value=0xa2)
+    def test_short_supplied_mac(self, mock_rnd):
+        mac_base = '12:34:56:78'
+        mac = mac_base.split(':')
+        generator = itertools.islice(net.random_mac_generator(mac), 1)
+        self.assertEqual(['12:34:56:78:a2:a2'], list(generator))
+        mock_rnd.assert_called_with(8)
+
 
 class TestPortDeviceOwner(base.BaseTestCase):
 
diff --git a/neutron_lib/utils/net.py b/neutron_lib/utils/net.py
index 9c5f36357..33870e24d 100644
--- a/neutron_lib/utils/net.py
+++ b/neutron_lib/utils/net.py
@@ -56,7 +56,7 @@ def random_mac_generator(base_mac):
     fixed = list(base_mac[0:3])
     to_generate = 3
     if base_mac[3] != '00':
-        fixed += base_mac[3]
+        fixed = list(base_mac[0:4])
         to_generate = 2
     beginning = ':'.join(fixed) + ':'