Hydra evaluation failure

[Leo Famulari <leo@famulari.name>]

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hydra-eval-guile-jobs returned exit code 1:
[...]
Backtrace:
In ice-9/eval.scm:
 432: 19 [eval # #]
In unknown file:
   ?: 18 [call-with-input-string "(apply (module-ref (resolve-interface '(hydra-eval-guile-jobs)) 'eval-guile-jobs) (cdr (command-line)))" ...]
In ice-9/command-line.scm:
 181: 17 [#<procedure 9b94c0 at ice-9/command-line.scm:176:6 (port)> #<input: string 1286c30>]
In unknown file:
   ?: 16 [eval (apply (module-ref # #) (cdr #)) #<directory (guile-user) 993bd0>]
In /usr/local/bin/hydra-eval-guile-jobs:
 157: 15 [job-evaluations->xml (# # # # ...) #<output: file 1> #:gc-roots-dir ...]
  88: 14 [job-evaluations->sxml (# # # ...) #:gc-roots-dir ...]
In srfi/srfi-1.scm:
 575: 13 [map #<procedure 126cfa20 at /usr/local/bin/hydra-eval-guile-jobs:88:12 (expr)> ...]
In /usr/local/bin/hydra-eval-guile-jobs:
  90: 12 [#<procedure 126cfa20 at /usr/local/bin/hydra-eval-guile-jobs:88:12 (expr)> #]
In ice-9/boot-9.scm:
2404: 11 [save-module-excursion #<procedure 25b34960 at /usr/local/bin/hydra-eval-guile-jobs:91:30 ()>]
In ice-9/r4rs.scm:
 176: 10 [with-output-to-port #<variable 236d88e0 value: #<output: file 1>> ...]
In ice-9/eval.scm:
 481: 9 [lp (#<fluid 38>) (#f)]
 387: 8 [eval # #]
 387: 7 [eval # #]
 387: 6 [eval # #]
In guix/packages.scm:
 817: 5 [cache! #<weak-key-hash-table 3cfbce0 14372/28099> # # ...]
1132: 4 [thunk]
 817: 3 [cache! #<weak-key-hash-table 3cfb8c0 14377/28099> # # ...]
 927: 2 [thunk]
In gnu/packages/mes.scm:
  79: 1 [native-inputs]
In unknown file:
   ?: 0 [string-prefix? #f "x86_64-linux" ...]

ERROR: In procedure string-prefix?:
ERROR: In procedure string-prefix?: Wrong type argument in position 1 (expecting string): #f

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