diff --git a/0001-Fix-_WIN32-is-not-defined-when-not-on-Windows.patch b/0001-Fix-_WIN32-is-not-defined-when-not-on-Windows.patch
new file mode 100644
index 0000000..33f8316
--- /dev/null
+++ b/0001-Fix-_WIN32-is-not-defined-when-not-on-Windows.patch
@@ -0,0 +1,25 @@
+From 7b1df6fc2b989d8a64edaa47fe04edea95c45174 Mon Sep 17 00:00:00 2001
+From: Stefan Gerlach <stefan.gerlach@uni-konstanz.de>
+Date: Thu, 25 Jul 2024 08:21:38 +0000
+Subject: [PATCH] Fix "_WIN32" is not defined when not on Windows.
+
+---
+ include/ixion/env.hpp | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/include/ixion/env.hpp b/include/ixion/env.hpp
+index 92812e3..7fee070 100644
+--- a/include/ixion/env.hpp
++++ b/include/ixion/env.hpp
+@@ -33,7 +33,7 @@
+   #define IXION_DLLPUBLIC_VAR IXION_DLLPUBLIC extern
+ #endif
+ 
+-#if _WIN32
++#ifdef _WIN32
+ #define IXION_MOD_EXPORT __declspec(dllexport)
+ #else
+ #define IXION_MOD_EXPORT __attribute__ ((visibility ("default")))
+-- 
+2.47.1
+
diff --git a/libixion.changes b/libixion.changes
index cf3eba2..1d4be5c 100644
--- a/libixion.changes
+++ b/libixion.changes
@@ -1,3 +1,9 @@
+-------------------------------------------------------------------
+Tue Jan 14 00:51:43 UTC 2025 - Christophe Marin <christophe@krop.fr>
+
+- Add upstream change to fix failure with -Werror=undef:
+  * 0001-Fix-_WIN32-is-not-defined-when-not-on-Windows.patch
+
 -------------------------------------------------------------------
 Thu Feb 29 16:51:10 UTC 2024 - pgajdos@suse.com
 
diff --git a/libixion.spec b/libixion.spec
index a41da2d..ba05729 100644
--- a/libixion.spec
+++ b/libixion.spec
@@ -26,6 +26,8 @@ License:        MIT
 URL:            https://gitlab.com/ixion/ixion
 Source:         http://kohei.us/files/ixion/src/%{name}-%{version}.tar.xz
 Patch0:         libixion-boost-system.patch
+# PATCH-FIX-UPSTREAM
+Patch1:         0001-Fix-_WIN32-is-not-defined-when-not-on-Windows.patch
 BuildRequires:  autoconf
 BuildRequires:  automake
 BuildRequires:  coreutils