--- a/session.c
+++ b/session.c
@@ -40,7 +40,7 @@
 #define	ASYNC_MODE	FNDELAY
 #elif	O_ASYNC
 #define	ASYNC_MODE	O_ASYNC
-#elif
+#else
 #error the fcntl argument to turn ON/OFF non-blocking I/O is unknown
 #endif