diff --git a/src/hostinfo/dhcpd.py b/src/hostinfo/dhcpd.py
index 1768cf3ad8b853a68b9e2cf071ae97541228620a..194a5fd1ad1687bbf6b905c5573c02d70f91e144 100755
--- a/src/hostinfo/dhcpd.py
+++ b/src/hostinfo/dhcpd.py
@@ -210,6 +210,7 @@ def emit_subnet(tree, n, dhcphost, kickstart, next_server=None):
         ether = i._parent.ethernet[0]
         ip = i.address[0]
         if ether:
+            assert ether.lower() == ether, "%s is not lower-case" % ether
             result += "    host %s.%s {\n"% (i.name[0:], n.domain[0])
             result += "      hardware ethernet %s;\n" % ether
             result += "      fixed-address %s;\n" % ip