diff --git a/guix/build/ruby-build-system.scm b/guix/build/ruby-build-system.scm
index c957a61115..8a421c1e15 100644
--- a/guix/build/ruby-build-system.scm
+++ b/guix/build/ruby-build-system.scm
@@ -2,6 +2,7 @@
 ;;; Copyright © 2015 David Thompson <davet@gnu.org>
 ;;; Copyright © 2015 Pjotr Prins <pjotr.public01@thebird.nl>
 ;;; Copyright © 2015, 2016 Ben Woodcroft <donttrustben@gmail.com>
+;;; Copyright © 2020 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -73,13 +74,19 @@ directory."
 
 (define* (replace-git-ls-files #:key source #:allow-other-keys)
   "Many gemspec files downloaded from outside rubygems.org use `git ls-files`
-to list of the files to be included in the built gem.  However, since this
+to list the files to be included in the built gem.  However, since this
 operation is not deterministic, we replace it with `find`."
-  (when (not (gem-archive? source))
+  (unless (gem-archive? source)
     (let ((gemspec (first-gemspec)))
+      ;; Do not include the freshly built .gem itself as it causes problems.
+      ;; Strip the first 2 characters ("./") to more exactly match the output
+      ;; given by 'git ls-files'.  This is useful to prevent breaking regexps
+      ;; that could be used to filter the list of files.
       (substitute* gemspec
-        (("`git ls-files`") "`find . -type f |sort`")
-        (("`git ls-files -z`") "`find . -type f -print0 |sort -z`"))))
+        (("`git ls-files`")
+         "`find . -type f -not -regex '.*\\.gem$' | sort | cut -c3-`")
+        (("`git ls-files -z`")
+         "`find . -type f -not -regex '.*\\.gem$' -print0 | sort -z | cut -zc3-`"))))
   #t)
 
 (define* (extract-gemspec #:key source #:allow-other-keys)