diff --git a/src/modbus-rtu-private.h b/src/modbus-rtu-private.h
index 221734f..93dfb03 100644
--- a/src/modbus-rtu-private.h
+++ b/src/modbus-rtu-private.h
@@ -38,6 +38,8 @@
 #define _MODBUS_RTU_CHECKSUM_LENGTH    2
 
 #if defined(_WIN32)
+#define ENOTSUP WSAEOPNOTSUPP
+
 /* WIN32: struct containing serial handle and a receive buffer */
 #define PY_BUF_SIZE 512
 struct win32_ser {