Not sure if this is a bug or misconfiguration.
Here is relevant part of my config.scm

(use-modules (gnu)
               (gnu system nss)
               (gnu services xorg))
(use-service-modules desktop)
(use-package-modules xfce ratpoison wicd avahi xorg certs)
;;;System definition cut away.
   (services (cons* (slim-service #:startx (xorg-start-command
#:configuration-file (xorg-configuration-file #:drivers '("radeon" "vesa")
                         #:resolutions '((1366 768) (1024 768)))))
                   ;;%desktop-services
                   %base-services))

But when I run gux system reconfigure /etc/config.scm I get error:

substitute: updating list of substitutes from 'http://hydra.gnu.org'...
100.0%
The following derivations will be built:
   /gnu/store/hap769hzynkhcqin9ynvjnsf4rdgphyv-system.drv
   /gnu/store/g285cghja69w8lamlir2w1w1w0axa35a-grub.cfg.drv
   /gnu/store/3x9qz66gpqisa18hzjbjj6rbm6cxfsq7-slim.cfg.drv
   /gnu/store/f9adqf84wq70lakxfmh4f4cn6xwhszdb-dmd.conf.drv
   /gnu/store/45fhs53hdy98j10h2bnm4ilqy70sg96i-boot.drv
ERROR: In procedure primitive-load:
ERROR: In procedure scm_lreadr:
/gnu/store/6bhrflbps4mav5yv5mas6fazcb59734f-slim.cfg-builder:1:159: Unknown
# object: #\<
builder for `/gnu/store/3x9qz66gpqisa18hzjbjj6rbm6cxfsq7-slim.cfg.drv'
failed with exit code 1
cannot build derivation
`/gnu/store/f9adqf84wq70lakxfmh4f4cn6xwhszdb-dmd.conf.drv': 1 dependencies
couldn't be built
cannot build derivation
`/gnu/store/45fhs53hdy98j10h2bnm4ilqy70sg96i-boot.drv': 1 dependencies
couldn't be built
cannot build derivation
`/gnu/store/hap769hzynkhcqin9ynvjnsf4rdgphyv-system.drv': 1 dependencies
couldn't be built
guix system: error: build failed: build of
`/gnu/store/hap769hzynkhcqin9ynvjnsf4rdgphyv-system.drv' failed

Please, help me set screen resolution on my laptop.Thanks in advance.

Dmitry