I get a *Messages* buffer like

| window-system: nil
| For information about GNU Emacs and the GNU system, type C-h C-a.

After starting up,

window-system
   ==> x

as expected.


Thanks,

Michael.


In GNU Emacs 27.0.50 (build 5, x86_64-pc-linux-gnu, GTK+ Version 3.22.29)
 of 2018-03-30 built on drachen
Repository revision: befd2667396125748d6e9e0e15c5fc38f7846082
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description: Debian GNU/Linux buster/sid