diff --git a/sys/conf/kern.post.mk b/sys/conf/kern.post.mk
--- a/sys/conf/kern.post.mk
+++ b/sys/conf/kern.post.mk
@@ -133,7 +133,7 @@
 all:
 .for __i in ${PORTS_MODULES}
 	@${ECHO} "===> Ports module ${__i} (all)"
-	cd $${PORTSDIR:-/usr/ports}/${__i}; ${PORTSMODULESENV} ${MAKE} -B clean build
+	cd ${PORTSDIR:U/usr/ports}/${__i}; ${PORTSMODULESENV} ${MAKE} -B clean build
 .endfor
 
 .for __target in install reinstall clean
@@ -141,7 +141,7 @@
 ports-${__target}:
 .for __i in ${PORTS_MODULES}
 	@${ECHO} "===> Ports module ${__i} (${__target})"
-	cd $${PORTSDIR:-/usr/ports}/${__i}; ${PORTSMODULESENV} ${MAKE} -B ${__target:C/(re)?install/deinstall reinstall/}
+	cd ${PORTSDIR:U/usr/ports}/${__i}; ${PORTSMODULESENV} ${MAKE} -B ${__target:C/(re)?install/deinstall reinstall/}
 .endfor
 .endfor
 .endif