diff --git a/lib/libtacplus/taclib.h b/lib/libtacplus/taclib.h
--- a/lib/libtacplus/taclib.h
+++ b/lib/libtacplus/taclib.h
@@ -60,6 +60,7 @@
 #define TAC_AUTHEN_SENDAUTH	0x04
 
 /* Authentication types */
+#define TAC_AUTHEN_TYPE_NOT_SET	0x00
 #define TAC_AUTHEN_TYPE_ASCII	0x01
 #define TAC_AUTHEN_TYPE_PAP	0x02
 #define TAC_AUTHEN_TYPE_CHAP	0x03