diff --git a/lib/c-stack.h b/lib/c-stack.h
index 56d74f1..8bf773c 100644
--- a/lib/c-stack.h
+++ b/lib/c-stack.h
@@ -34,8 +34,10 @@
    A null ACTION acts like an action that does nothing.
 
    ACTION must be async-signal-safe.  ACTION together with its callees
-   must not require more than 64 KiB of stack space.  Also,
-   ACTION should not call longjmp, because this implementation does
+   must not require more than 64 KiB of stack space.  ACTION must not create
+   nested functions <https://gcc.gnu.org/onlinedocs/gcc/Nested-Functions.html>,
+   because this implementation does not guarantee an executable stack.
+   Also, ACTION should not call longjmp, because this implementation does
    not guarantee that it is safe to return to the original stack.
 
    This function may install a handler for the SIGSEGV signal or for the SIGBUS