diff --git a/test-assets/puzzles/notify-unknown-open-issues.xml b/test-assets/puzzles/notify-unknown-open-issues.xml
new file mode 100644
index 00000000..92811192
--- /dev/null
+++ b/test-assets/puzzles/notify-unknown-open-issues.xml
@@ -0,0 +1,37 @@
+<?xml version="1.0"?>
+<!--
+(The MIT License)
+
+Copyright (c) 2016-2022 Yegor Bugayenko
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the 'Software'), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+-->
+<puzzles>
+  <puzzle alive="true">
+    <id>1-abcdef</id>
+    <issue>5</issue>
+    <children>
+      <puzzle alive="true">
+        <id>5-abcdef</id>
+        <issue href="//issue/125">unknown</issue>
+        <ticket>5</ticket>
+      </puzzle>
+    </children>
+  </puzzle>
+</puzzles>
diff --git a/test/test_diff.rb b/test/test_diff.rb
index 8bebb7d0..4cddb607 100644
--- a/test/test_diff.rb
+++ b/test/test_diff.rb
@@ -60,6 +60,22 @@ def test_notification_on_one_new_puzzle
     )
   end
 
+  def test_notification_unknown_issue
+    tickets = Tickets.new
+    xml = File.open('test-assets/puzzles/notify-unknown-open-issues.xml') do |f|
+      Nokogiri::XML(f)
+    end
+    Diff.new(Nokogiri::XML('<puzzles/>'), xml).notify(tickets)
+    assert(
+      tickets.messages.length == 1,
+      "Incorrect number of messages: #{tickets.messages.length}"
+    )
+    assert(
+      tickets.messages[0] == '5 the puzzle [#125](//issue/125) is still not solved.',
+      "Text is wrong: #{tickets.messages[0]}"
+    )
+  end
+
   def test_notification_on_two_new_puzzles
     tickets = Tickets.new
     Diff.new(