diff --git a/hooks/debian/20-setup-apt b/hooks/debian/20-setup-apt
index 0d9353f..9c8189b 100755
--- a/hooks/debian/20-setup-apt
+++ b/hooks/debian/20-setup-apt
@@ -76,9 +76,11 @@ E_O_APT
 
 
 #
-#  If the host system has security support then enable that here too.
+#  If the host system has security support then enable that here, too,
+#  except if we're installing Debian Unstable.
 #
-if ( test -e /etc/apt/sources.list && \
+if ( test "${dist}" '!=' 'sid' && test "${dist}" '!=' 'unstable' && \
+     test -e /etc/apt/sources.list && \
      grep ^deb.*security -r /etc/apt/sources.list /etc/apt/sources.list.d >/dev/null 2>/dev/null ) ; then
 
     cat <<E_O_APT >> ${prefix}/etc/apt/sources.list