diff --git a/mrbgems/hal-win-socket/src/socket_hal.c b/mrbgems/hal-win-socket/src/socket_hal.c
index 9b28d324a..3acda0b5e 100644
--- a/mrbgems/hal-win-socket/src/socket_hal.c
+++ b/mrbgems/hal-win-socket/src/socket_hal.c
@@ -20,6 +20,7 @@
 #include <winsock2.h>
 #include <ws2tcpip.h>
 #include <windows.h>
+#include <errno.h>
 #include <string.h>
 
 /*