diff --git a/management/dns_update.py b/management/dns_update.py
index 9f664172..6d2c957e 100755
--- a/management/dns_update.py
+++ b/management/dns_update.py
@@ -918,7 +918,7 @@ def set_secondary_dns(hostnames, env):
 							raise ValueError("That's neither an IPv4 or IPv6 subnet.")
 					else:
 						v = ipaddress.ip_address(item[4:]) # raises a ValueError if there's a problem
-						if not isinstance(v, ipaddress.IPv4Network) and not isinstance(v, ipaddress.IPv6Network):
+						if not isinstance(v, ipaddress.IPv4Address) and not isinstance(v, ipaddress.IPv6Address):
 							raise ValueError("That's neither an IPv4 or IPv6 address.")
 				except ValueError:
 					raise ValueError("'%s' is not an IPv4 or IPv6 address or subnet." % item[4:])