diff --git a/source/Platform/Win32/Win32Display.cpp b/source/Platform/Win32/Win32Display.cpp
index 67993fa..887688c 100644
--- a/source/Platform/Win32/Win32Display.cpp
+++ b/source/Platform/Win32/Win32Display.cpp
@@ -12,6 +12,11 @@ namespace sp {
 
 #define WND_CLASSNAME "SPECTRE_WIN32_WNDCLASS"
 
+#if defined(_WIN64) && !defined(GWL_USERDATA)
+// x64 undefines this. So we define it again.
+#define GWL_USERDATA GWLP_USERDATA
+#endif /* defined(_WIN64) && !defined(GWL_USERDATA) */
+
 static bool firstTime = true;
 
 Win32Display::Win32Display() :