diff --git a/nova/block_device.py b/nova/block_device.py
index cf670a6486a5..38e1c62f80c3 100644
--- a/nova/block_device.py
+++ b/nova/block_device.py
@@ -494,7 +494,7 @@ _pref = re.compile('^((x?v|s|h)d)')
 def strip_prefix(device_name):
     """remove both leading /dev/ and xvd or sd or vd or hd."""
     device_name = strip_dev(device_name)
-    return _pref.sub('', device_name)
+    return _pref.sub('', device_name) if device_name else device_name
 
 
 _nums = re.compile('\d+')
@@ -504,7 +504,7 @@ def get_device_letter(device_name):
     letter = strip_prefix(device_name)
     # NOTE(vish): delete numbers in case we have something like
     #             /dev/sda1
-    return _nums.sub('', letter)
+    return _nums.sub('', letter) if device_name else device_name
 
 
 def instance_block_mapping(instance, bdms):
diff --git a/nova/tests/unit/test_block_device.py b/nova/tests/unit/test_block_device.py
index 5e9625c484d6..2777cbc67013 100644
--- a/nova/tests/unit/test_block_device.py
+++ b/nova/tests/unit/test_block_device.py
@@ -125,6 +125,7 @@ class BlockDeviceTestCase(test.NoDBTestCase):
     def test_strip_dev(self):
         self.assertEqual('sda', block_device.strip_dev('/dev/sda'))
         self.assertEqual('sda', block_device.strip_dev('sda'))
+        self.assertIsNone(block_device.strip_dev(None))
 
     def test_strip_prefix(self):
         self.assertEqual('a', block_device.strip_prefix('/dev/sda'))
@@ -132,6 +133,7 @@ class BlockDeviceTestCase(test.NoDBTestCase):
         self.assertEqual('a', block_device.strip_prefix('xvda'))
         self.assertEqual('a', block_device.strip_prefix('vda'))
         self.assertEqual('a', block_device.strip_prefix('hda'))
+        self.assertIsNone(block_device.strip_prefix(None))
 
     def test_get_device_letter(self):
         self.assertEqual('', block_device.get_device_letter(''))
@@ -142,6 +144,7 @@ class BlockDeviceTestCase(test.NoDBTestCase):
         self.assertEqual('b', block_device.get_device_letter('sdb2'))
         self.assertEqual('c', block_device.get_device_letter('vdc'))
         self.assertEqual('c', block_device.get_device_letter('hdc'))
+        self.assertIsNone(block_device.get_device_letter(None))
 
     def test_volume_in_mapping(self):
         swap = {'device_name': '/dev/sdb',