diff --git a/lib/krb5/krb5.h b/lib/krb5/krb5.h
index 8fc65ea13..c51034090 100644
--- a/lib/krb5/krb5.h
+++ b/lib/krb5/krb5.h
@@ -208,7 +208,8 @@ typedef enum krb5_address_type {
 
 enum {
   AP_OPTS_USE_SESSION_KEY = 1,
-  AP_OPTS_MUTUAL_REQUIRED = 2
+  AP_OPTS_MUTUAL_REQUIRED = 2,
+  AP_OPTS_USE_SUBKEY = 4		/* library internal */
 };
 
 typedef HostAddress krb5_address;