diff --git a/dataapi.py b/dataapi.py
index c05a819833327d09b91f75bcdd2c92fdc8fef553..da691388bbe0e9593b7be8e14e69090ca55c24e1 100644
--- a/dataapi.py
+++ b/dataapi.py
@@ -15,9 +15,9 @@ def load_nsname2ipset() -> Dict[dns.name.Name, Set[AnIPAddress]]:
     logging.info('loading NS name -> IP address mapping')
     with open('nsname2ipset.pickle', 'rb') as nsname2ipset_pickle:
         nsname2ipset = pickle.load(nsname2ipset_pickle)
-    ip_all = set.union(*nsname2ipset.values())
+    ip_cnt = functools.reduce(lambda cnt, ipset: cnt + len(ipset), nsname2ipset.values(), 0)
     logging.info('loaded %s unique IP addresses for %s NS names',
-                 len(ip_all), len(nsname2ipset))
+                 ip_cnt, len(nsname2ipset))
     return nsname2ipset
 
 def load_domain2ipset() -> Dict[dns.name.Name, Set[AnIPAddress]]: