diff --git a/src/address.cc b/src/address.cc
index 3168f70..1c8498c 100644
--- a/src/address.cc
+++ b/src/address.cc
@@ -20,6 +20,7 @@
 #include <cstring>
 #include <cstdio>
 #include <cstdlib>
+#include <cctype>
 
 #include <netinet/ip6.h>
 #include <arpa/inet.h>