diff --git a/win32/Makefile.sub b/win32/Makefile.sub
index e2d9d6159f..e07ed93b60 100644
--- a/win32/Makefile.sub
+++ b/win32/Makefile.sub
@@ -586,6 +586,7 @@ $(CONFIG_H): $(MKFILES) $(srcdir)/win32/Makefile.sub $(win_srcdir)/Makefile.sub
 #define DEPRECATED(x) __declspec(deprecated) x
 #define NOINLINE(x) __declspec(noinline) x
 !endif
+#define UNREACHABLE __assume(0)
 #define FUNC_STDCALL(x) __stdcall x
 #define FUNC_CDECL(x) __cdecl x
 #define FUNC_FASTCALL(x) __fastcall x