Even simpler:

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$ ./pre-inst-env  guile ~/src/guix-debugging/http-pipelining-gzip.scm

;;; (port #<input-output: file 7fced4c20000>)

;;; (input #<input: file 7fced4c8ef50>)

;;; (drain "")

;;; (fetch 16397)

;;; (port #<input-output: file 7fced4c20000>)
Backtrace:
In ice-9/boot-9.scm:
  1736:10 11 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
In unknown file:
          10 (apply-smob/0 #<thunk 7fced7866520>)
In ice-9/boot-9.scm:
    718:2  9 (call-with-prompt _ _ #<procedure default-prompt-handler (k proc)>)
In ice-9/eval.scm:
    619:8  8 (_ #(#(#<directory (guile-user) 7fced7869c80>)))
In ice-9/boot-9.scm:
   2806:4  7 (save-module-excursion _)
  4351:12  6 (_)
In /home/ludo/src/guix-debugging/http-pipelining-gzip.scm:
     18:4  5 (_)
In guix/http-client.scm:
   116:21  4 (http-fetch _ #:port _ #:text? _ #:buffered? _ #:open-connection _ #:keep-alive? _ # _ #:headers _ # _)
In web/client.scm:
   526:24  3 (http-request _ #:body _ #:verify-certificate? _ #:port _ #:method _ #:version _ #:keep-alive? _ # _ # …)
In web/response.scm:
   198:31  2 (read-response #<input-output: string 7fced4c20000>)
In web/http.scm:
  1194:12  1 (read-response-line _)
    252:2  0 (parse-http-version _ _ _)

web/http.scm:252:2: In procedure parse-http-version:
Bad http-version header component: <!DOCTYPE

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