diff --git a/tools/python/xen/xend/server/blkif.py b/tools/python/xen/xend/server/blkif.py
index d6e449e337..dd788da10d 100644
--- a/tools/python/xen/xend/server/blkif.py
+++ b/tools/python/xen/xend/server/blkif.py
@@ -80,6 +80,9 @@ class BlkifController(DevController):
                          'acm_policy' : policy})
 
         devid = blkif.blkdev_name_to_number(dev)
+        if not devid:
+            raise VmError('Unable to find number for device (%s)' % (dev))
+
         front = { 'virtual-device' : "%i" % devid,
                   'device-type' : dev_type
                 }