diff --git a/source/lib/sysdep/os/win/acpi.cpp b/source/lib/sysdep/os/win/acpi.cpp
index fa91e3a3dd..874200d547 100644
--- a/source/lib/sysdep/os/win/acpi.cpp
+++ b/source/lib/sysdep/os/win/acpi.cpp
@@ -1,4 +1,4 @@
-/* Copyright (C) 2024 Wildfire Games.
+/* Copyright (C) 2025 Wildfire Games.
  *
  * Permission is hereby granted, free of charge, to any person obtaining
  * a copy of this software and associated documentation files (the
@@ -29,6 +29,7 @@
 #include "lib/sysdep/os/win/wfirmware.h"
 
 #include <atomic>
+#include <cctype>
 
 #pragma pack(1)