diff --git a/shadowsocks/common.py b/shadowsocks/common.py
index 0f94e07..68ca025 100644
--- a/shadowsocks/common.py
+++ b/shadowsocks/common.py
@@ -226,12 +226,12 @@ class IPNetwork(object):
         addr_family = is_ip(addr)
         if addr_family is socket.AF_INET:
             ip, = struct.unpack("!I", socket.inet_aton(addr))
-            return any(map(lambda (n, ps): n == ip >> ps,
+            return any(map(lambda n_ps: n_ps[0] == ip >> n_ps[1],
                            self._network_list_v4))
         elif addr_family is socket.AF_INET6:
             hi, lo = struct.unpack("!QQ", inet_pton(addr_family, addr))
             ip = (hi << 64) | lo
-            return any(map(lambda (n, ps): n == ip >> ps,
+            return any(map(lambda n_ps: n_ps[0] == ip >> n_ps[1],
                            self._network_list_v6))
         else:
             return False