Use the new Py_ARRAY_LENGTH macro
diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index dea2149..3576ced 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -1771,7 +1771,7 @@
         the argument must be the full path anyway. */
         wchar_t *ptemp;
         if (GetFullPathNameW(argv0,
-                           sizeof(fullpath)/sizeof(fullpath[0]),
+                           Py_ARRAY_LENGTH(fullpath),
                            fullpath,
                            &ptemp)) {
             argv0 = fullpath;