diff --git a/src/include/86box/serial.h b/src/include/86box/serial.h
index f580f9f9f..576c0c12e 100644
--- a/src/include/86box/serial.h
+++ b/src/include/86box/serial.h
@@ -39,6 +39,8 @@
 #define SERIAL4_ADDR		0x02e8
 #define SERIAL4_IRQ		3
 
+#define MAX_SERIAL		4
+
 
 struct serial_device_s;
 struct serial_s;