`guix package -r pgk` fails

[Ricardo Wurmus]

I can not remove packages with the latest guix from git:

~~~
address@hidden:~/code/guix] (526) $ ./pre-inst-env guix package -r samtools
The following package will be removed:
   samtools     1.1     /gnu/store/7jyc8p4cl7mfls9wq3fb7f6m6ipjvgd9-samtools-1.1

Backtrace:
In ice-9/boot-9.scm:
  63: 19 [call-with-prompt prompt0 ...]
In ice-9/eval.scm:
 432: 18 [eval # #]
In ice-9/boot-9.scm:
2401: 17 [save-module-excursion #<procedure 132b940 at ice-9/boot-9.scm:4045:3 
()>]
4050: 16 [#<procedure 132b940 at ice-9/boot-9.scm:4045:3 ()>]
1724: 15 [%start-stack load-stack ...]
1729: 14 [#<procedure 1343ea0 ()>]
In unknown file:
   ?: 13 [primitive-load "/home/rwurmus/code/guix/scripts/guix"]
In guix/ui.scm:
 781: 12 [run-guix-command package "-r" "samtools"]
In ice-9/boot-9.scm:
 157: 11 [catch srfi-34 #<procedure 258a960 at guix/ui.scm:218:2 ()> ...]
 157: 10 [catch system-error ...]
In guix/scripts/package.scm:
 975: 9 [#<procedure 258ec00 at guix/scripts/package.scm:964:4 ()>]
 821: 8 [process-actions (# # # # ...)]
In guix/ui.scm:
 311: 7 [show-what-to-build #<build-daemon 256.14 2593ec0> (#) #:dry-run? ...]
In guix/utils.scm:
 681: 6 [loop () () ...]
In guix/ui.scm:
 313: 5 [#<procedure 3892630 at guix/ui.scm:311:24 (drv build download)> # () 
()]
In guix/derivations.scm:
 206: 4 [derivation-prerequisites-to-build # # # ...]
In guix/store.scm:
 694: 3 [substitutable-paths # #]
In srfi/srfi-1.scm:
 616: 2 [for-each #<procedure 319e2c0 at guix/serialization.scm:112:12 
(t-6257)> #]
In guix/serialization.scm:
  85: 1 [write-string #f #<input-output: socket 10>]
In unknown file:
   ?: 0 [string->utf8 #f]

ERROR: In procedure string->utf8:
ERROR: In procedure string->utf8: Wrong type argument in position 1 (expecting 
string): #f
~~~

I have only two packages installed: samtools and emacs.  Attempting to
remove either package results in this error.

I tried previous versions from git (going back as far as the tagged
v0.8), but there's no difference.  I suppose it's something to do with
my system.

LANG is set to en_GB.UTF-8.  I'm running guix on top of Fedora 20.

Best,
Ricardo