diff --git a/tool/sync_default_gems.rb b/tool/sync_default_gems.rb
index 40da50a86b2b9c..15f651ca06e435 100755
--- a/tool/sync_default_gems.rb
+++ b/tool/sync_default_gems.rb
@@ -466,13 +466,13 @@ def message_filter(repo, sha, input: ARGF)
     if log and !log.empty?
       log.sub!(/(?<=\n)\n+\z/, '') # drop empty lines at the last
       conv[log]
-      log.sub!(/(?:(\A\s*)|\s*\n)(?=((?i:Co-authored-by:.*\n)+)?\Z)/) {
+      log.sub!(/(?:(\A\s*)|\s*\n)(?=((?i:^Co-authored-by:.*\n?)+)?\Z)/) {
         ($~.begin(1) ? "" : "\n\n") + commit_url + ($~.begin(2) ? "\n" : "")
       }
     else
       log = commit_url
     end
-    print subject, "\n\n", log
+    puts subject, "\n", log
   end
 
   # NOTE: This method is also used by GitHub ruby/git.ruby-lang.org's bin/update-default-gem.sh
diff --git a/tool/test/test_sync_default_gems.rb b/tool/test/test_sync_default_gems.rb
index 602598c3e34304..3987b1bec5414a 100755
--- a/tool/test/test_sync_default_gems.rb
+++ b/tool/test/test_sync_default_gems.rb
@@ -62,5 +62,15 @@ def test_multiple_co_authored_by
       ]
       assert_message_filter(expected, trailers, [expected, "", trailers, ""].join("\n"))
     end
+
+    def test_co_authored_by_no_newline
+      expected = [
+        "commit something",
+      ]
+      trailers = [
+        "Co-Authored-By: git <git@ruby-lang.org>",
+      ]
+      assert_message_filter(expected, trailers, [expected, "", trailers].join("\n"))
+    end
   end
 end