build failed: substituter `substitute' died unexpectedly

[Christopher Allan Webber]

I am getting this error right now on multiple machines:

  substitute: updating list of substitutes from 
'http://mirror.hydra.gnu.org'... 100.0%
  substitute: updating list of substitutes from 
'http://mirror.hydra.gnu.org'... 100.0%
  substitute: updating list of substitutes from 
'http://mirror.hydra.gnu.org'...  66.7%Backtrace:
  substitute: In ice-9/boot-9.scm:
  substitute:  160: 9 [catch #t #<catch-closure 1d5ba00> ...]
  substitute: In unknown file:
  substitute:    ?: 8 [apply-smob/1 #<catch-closure 1d5ba00>]
  substitute: In ice-9/boot-9.scm:
  substitute:   66: 7 [call-with-prompt prompt0 ...]
  substitute: In ice-9/eval.scm:
  substitute:  432: 6 [eval # #]
  substitute: In ice-9/boot-9.scm:
  substitute: 2412: 5 [save-module-excursion #<procedure 1d7c900 at 
ice-9/boot-9.scm:4084:3 ()>]
  substitute: 4089: 4 [#<procedure 1d7c900 at ice-9/boot-9.scm:4084:3 ()>]
  substitute: 1734: 3 [%start-stack load-stack ...]
  substitute: 1739: 2 [#<procedure 1d8eb40 ()>]
  substitute: In unknown file:
  substitute:    ?: 1 [primitive-load 
"/gnu/store/nqy9m6hhnkkfwr5wyq5bac96v9s9hc9i-guix-0.12.0-9.25a4/bin/.guix-real"]
  substitute: In guix/ui.scm:
  substitute: 1255: 0 [run-guix-command substitute "--query"]
  substitute:
  substitute: guix/ui.scm:1255:8: In procedure run-guix-command:
  substitute: guix/ui.scm:1255:8: Throw to key `bad-response' with args `("Bad 
Response-Line: ~s" (""))'.
  guix environment: error: build failed: substituter `substitute' died 
unexpectedly
  
  Some deprecated features have been used.  Set the environment
  variable GUILE_WARN_DEPRECATED to "detailed" and rerun the
  program to get more information.  Set it to "no" to suppress
  this message.

It doesn't seem to be caused by either the guix system revision or the
userspace guix version I'm running.  This happens with any operations
that use substitutes right now.

Are other people experiencing this?  What happened, and what's causing
it... do we know?