diff --git a/TODO.md b/TODO.md
index e9725698..426e2126 100644
--- a/TODO.md
+++ b/TODO.md
@@ -21,6 +21,7 @@
 - settle differences between intmax_t and uintmax_t
 - settle onDestroy/destroy naming
 - clean up Windows lifetiming code
+- forbid free(NULL) to check for bugs
 
 ultimately:
 - make everything vtable-based