diff -r 44670f6925d6 Makefile
--- a/Makefile	Fri Dec 02 23:12:37 2011 +0200
+++ b/Makefile	Sat Dec 03 01:05:50 2011 +0100
@@ -2,6 +2,10 @@
 .include <bsd.own.mk>
 
 FREEBSD_SRC_TREE?=/usr/src
+.if !exists(${FREEBSD_SRC_TREE}/sys/sys) 
+.error "missing FreeBSD sources: FREEBSD_SRC_TREE=${FREEBSD_SRC_TREE} (perhaps ${FREEBSD_SRC_TREE}/head ?)"
+.endif
+
 OBJ_DIR?=/usr/obj
 
 # ZROUTER_ROOT can be set in environment