diff --git a/nmap_config.h.in b/nmap_config.h.in
index 38617f27b..6827d7b62 100644
--- a/nmap_config.h.in
+++ b/nmap_config.h.in
@@ -237,5 +237,6 @@ extern "C" int gethostname (char *, unsigned int);
 #undef MACOSX
 
 #undef HAVE_IPV6_IPPROTO_RAW
+#undef PCAP_NETMASK_UNKNOWN
 
 #endif /* CONFIG_H */
diff --git a/nsock/include/nsock_config.h.in b/nsock/include/nsock_config.h.in
index 8279c2924..4bc334f11 100644
--- a/nsock/include/nsock_config.h.in
+++ b/nsock/include/nsock_config.h.in
@@ -85,3 +85,5 @@
 #undef HAVE_EPOLL
 #undef HAVE_POLL
 #undef HAVE_KQUEUE
+
+#undef PCAP_NETMASK_UNKNOWN