diff --git a/lladd/constants.h b/lladd/constants.h
index 9cd99a9..b64a767 100644
--- a/lladd/constants.h
+++ b/lladd/constants.h
@@ -158,6 +158,8 @@ terms specified in this license.
 
 //#define SLOT_TYPE_BASE PAGE_SIZE
 
+#define INVALID_XID (-1)
+
 /** This constant is used as a placeholder to mark slot locations that are invalid.
     @see slotted.c, indirect.c
 */