diff --git a/libc/include/sys/ipc.h b/libc/include/sys/ipc.h
index c81ec1a..2e2b8cf 100644
--- a/libc/include/sys/ipc.h
+++ b/libc/include/sys/ipc.h
@@ -52,6 +52,6 @@
  *
  * Returns a key on success, and returns -1 and sets `errno` on failure.
  */
-key_t ftok(const char* __path, int __id);
+key_t ftok(const char* _Nonnull __path, int __id);
 
 __END_DECLS