diff --git a/src/socket_base.cpp b/src/socket_base.cpp
index 263df8f5..ed7ccaf5 100644
--- a/src/socket_base.cpp
+++ b/src/socket_base.cpp
@@ -45,6 +45,7 @@
 #endif
 #else
 #include <unistd.h>
+#include <ctype.h>
 #endif
 
 #include "socket_base.hpp"
diff --git a/src/tcp_address.cpp b/src/tcp_address.cpp
index 5f2664d4..1c3de004 100644
--- a/src/tcp_address.cpp
+++ b/src/tcp_address.cpp
@@ -44,6 +44,7 @@
 #include <arpa/inet.h>
 #include <netinet/tcp.h>
 #include <netdb.h>
+#include <ctype.h>
 #endif
 
 #ifdef ZMQ_HAVE_SOLARIS