Hey,

I am facing a problem with (openvpn-client-service) when trying to
reconfigure the system.
Its GuixSD on a x86-64, the system config is attached.

Backtrace:
In srfi/srfi-1.scm:
   592:29 19 (map1 (#<<service> type: #<service-type nscd 22f74b0…> …))
   592:29 18 (map1 (#<<service> type: #<service-type guix 22f73c0…> …))
   592:29 17 (map1 (#<<service> type: #<service-type urandom-seed…> …))
   592:29 16 (map1 (#<<service> type: #<service-type syslog 22f74…> …))
   592:29 15 (map1 (#<<service> type: #<service-type static-netwo…> …))
   592:29 14 (map1 (#<<service> type: #<service-type mingetty 22f…> …))
   592:29 13 (map1 (#<<service> type: #<service-type mingetty 22f…> …))
   592:29 12 (map1 (#<<service> type: #<service-type mingetty 22f…> …))
   592:29 11 (map1 (#<<service> type: #<service-type mingetty 22f…> …))
   592:29 10 (map1 (#<<service> type: #<service-type mingetty 22f…> …))
   592:29  9 (map1 (#<<service> type: #<service-type mingetty 22f…> …))
   592:29  8 (map1 (#<<service> type: #<service-type console-font…> …))
   592:17  7 (map1 (#<<service> type: #<service-type openvpn-clie…> …))
In gnu/services/vpn.scm:
   409:24  6 (_ #<<openvpn-client-configuration> openvpn: #<package …>)
    379:9  5 (openvpn-config-file client #<<openvpn-client-configura…>)
In ice-9/ports.scm:
    549:4  4 (call-with-output-string _)
    473:4  3 (with-output-to-port _ _)
In ice-9/boot-9.scm:
   268:13  2 (for-each #<procedure 2636d40 at gnu/services/configur…> …)
In gnu/services/vpn.scm:
   112:19  1 (serialize-tls-auth client #f)
In unknown file:
           0 (string-append #f " " "1")

ERROR: In procedure string-append:
ERROR: In procedure string-append: Wrong type (expecting string): #f