From efd8617e3667e9ad859a04a920fd41d77374306a Mon Sep 17 00:00:00 2001
From: patchwright <292882882+patchwright@users.noreply.github.com>
Date: Sun, 14 Jun 2026 19:48:48 +0200
Subject: [PATCH] Fix Mongolian ordinal IndexError on zero
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num2words(0, lang='mn', to='ordinal') and to='ordinal_num' raised

IndexError because _get_ordinal_suffix indexed number_str[-2] on the

single-character string '0'. The base contract accepts 0 (English

returns 'zeroth'/'0th'; Dutch was fixed the same way in #365), so the

Mongolian module should honour it too. Guard the tens-digit branch with

a length check so 0 falls through to the default 'дугаар' suffix.
---
 num2words/lang_MN.py |  2 +-
 tests/test_mn.py     | 16 ++++++++++++++++
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/num2words/lang_MN.py b/num2words/lang_MN.py
index d7cc3d40..e38bd9e8 100644
--- a/num2words/lang_MN.py
+++ b/num2words/lang_MN.py
@@ -240,7 +240,7 @@ def _get_ordinal_suffix(self, number):
         if number_str[-1] != '0':
             if number_str[-1] in ('1', '4', '9'):
                 suffix = 'дүгээр'
-        elif number_str[-2] != '0':
+        elif len(number_str) > 1 and number_str[-2] != '0':
             if number_str[-2] in ('4', '9'):
                 suffix = 'дүгээр'
         return suffix
diff --git a/tests/test_mn.py b/tests/test_mn.py
index 79c9ff87..a8bfc005 100644
--- a/tests/test_mn.py
+++ b/tests/test_mn.py
@@ -86,6 +86,15 @@ def test_to_ordinal(self):
             "нэг мянга дугаар"
         )
 
+    def test_to_ordinal_zero(self):
+        # ordinal(0) must not crash; base.verify_ordinal accepts 0 (English
+        # returns "zeroth") and Mongolian should honour the same contract,
+        # falling back to the default "дугаар" suffix.
+        self.assertEqual(
+            num2words(0, lang='mn', to='ordinal'),
+            "тэг дугаар"
+        )
+
     def test_to_ordinal_num(self):
         self.assertEqual(
             num2words(105, lang='mn', to='ordinal_num'),
@@ -97,6 +106,13 @@ def test_to_ordinal_num(self):
             "489 дүгээр"
         )
 
+    def test_to_ordinal_num_zero(self):
+        # ordinal_num(0) must not crash; see test_to_ordinal_zero.
+        self.assertEqual(
+            num2words(0, lang='mn', to='ordinal_num'),
+            "0 дугаар"
+        )
+
     def test_to_year(self):
         self.assertEqual(
             num2words(2005, lang='mn', to="year"),