bug#25775: Can't install packages after guix pull

[Pjotr Prins <pjotr.public12@thebird.nl>]

And this appears to be related:

./pre-inst-env guix import cpan Time::ParseDate

Starting download of /tmp/guix-file.4UM1ZV
From http://mirror.ibcp.fr/pub/CPAN/authors/id/M/MU/MUIR/modules/Time-ParseDate-2015.103.tar.gz...
 ...2015.103.tar.gz  26KiB          214KiB/s 00:00 [####################] 100.0%
Backtrace:
In guix/packages.scm:
 982: 19 [bag-grafts # #]
 966: 18 [fold-bag-dependencies #<procedure 54aef20 at guix/packages.scm:982:29 (package grafts)> ...]
 983: 17 [#<procedure 54aef20 at guix/packages.scm:982:29 (package grafts)> # ()]
 786: 16 [cache! #<weak-key-hash-table 2be7100 0/223> # "x86_64-linux" ...]
 910: 15 [thunk]
 786: 14 [cache! #<weak-key-hash-table 2be7540 167/223> # # ...]
1092: 13 [thunk]
1024: 12 [bag->derivation # # #]
In srfi/srfi-1.scm:
 578: 11 [map #<procedure 5b16ba0 at guix/packages.scm:1026:30 (t-23184)> #]
In guix/packages.scm:
 846: 10 [expand-input # # # ...]
In guix/store.scm:
1249: 9 [run-with-store # ...]
In guix/packages.scm:
 480: 8 [#<procedure 5b3b870 at guix/packages.scm:480:2 (state)> #]
 485: 7 [#<procedure 420b600 at guix/packages.scm:485:34 (state)> #]
In gnu/packages/bash.scm:
 249: 6 [#<procedure 420b640 at gnu/packages/bash.scm:249:2 (state)> #]
In guix/download.scm:
 421: 5 [#<procedure 597a500 at guix/download.scm:421:2 (state)> #]
In guix/store.scm:
1169: 4 [#<procedure 5b3b720 at guix/store.scm:1168:28 (store)> #]
In guix/packages.scm:
 786: 3 [cache! #<weak-key-hash-table 2be7540 167/223> #f ...]
1092: 2 [thunk]
 786: 1 [cache! #<weak-key-hash-table 2be7240 170/223> #f ...]
 871: 0 [thunk]

guix/packages.scm:871:27: In procedure thunk:
guix/packages.scm:871:27: In procedure struct_vtable: Wrong type argument in position 1 (expecting struct): #f