diff --git a/stdlib/cxa_atexit.c b/stdlib/cxa_atexit.c
index 6d65f7e615..978f873990 100644
--- a/stdlib/cxa_atexit.c
+++ b/stdlib/cxa_atexit.c
@@ -36,6 +36,9 @@ __internal_atexit (void (*func) (void *), void *arg, void *d,
 {
   struct exit_function *new;
 
+  /* BZ#20544  */
+  assert (func != NULL);
+
   __libc_lock_lock (__exit_funcs_lock);
   new = __new_exitfn (listp);
 
diff --git a/stdlib/on_exit.c b/stdlib/on_exit.c
index 5241e0d86f..0ead1eafd4 100644
--- a/stdlib/on_exit.c
+++ b/stdlib/on_exit.c
@@ -15,6 +15,7 @@
    License along with the GNU C Library; if not, see
    <http://www.gnu.org/licenses/>.  */
 
+#include <assert.h>
 #include <stdlib.h>
 #include "exit.h"
 #include <sysdep.h>
@@ -25,6 +26,9 @@ __on_exit (void (*func) (int status, void *arg), void *arg)
 {
   struct exit_function *new;
 
+  /* BZ#20544  */
+  assert (func != NULL);
+
    __libc_lock_lock (__exit_funcs_lock);
   new = __new_exitfn (&__exit_funcs);