diff --git a/include/_tail_cc/sys/dir.h b/include/_tail_cc/sys/dir.h
index 11b210492..6f3573c06 100644
--- a/include/_tail_cc/sys/dir.h
+++ b/include/_tail_cc/sys/dir.h
@@ -3,7 +3,7 @@
  * (c) copyright 1987 by the Vrije Universiteit, Amsterdam, The Netherlands.
  * See the copyright notice in the ACK home directory, in the file "Copyright".
  */
-#ifdef BSD4_2
+#ifdef __BSD4_2
 #define MAXNAMLEN 255
 #else
 #define MAXNAMLEN 14