make distclean fails for conditional nested package with VPATH build

[Daily, Jeff A]

I had a user point out that "make distclean" from within their VPATH build 
failed due to a missing, optional nested package.  The nested package is called 
"tascel".  I have in configure.ac:

AC_ARG_ENABLE([tascel], [omit for brevity], [], [enable_tascel=no])
AM_CONDITIONAL([ENABLE_TASCEL], [test "x$enable_tascel" = xyes])
...
AS_IF([test "x$enable_tascel" = xyes], [AC_CONFIG_SUBDIRS([tascel])])

Then in Makefile.am:

SUBDIRS = armci .
If ENABLE_TASCEL
SUBDIRS += tascel
endif

I'm sure the Makefile is correct because we want to distributed tascel whether 
or not it's been configured.
SUBDIRS = armci .
DIST_SUBDIRS = armci . tascel

Do I tell this user "don't do that" or did I do something wrong?

Jeff