diff --git a/lib/libc/tests/stdlib/dynthr_mod/dynthr_mod.c b/lib/libc/tests/stdlib/dynthr_mod/dynthr_mod.c
index 4f5622f47a81..c19a1a54a8b6 100644
--- a/lib/libc/tests/stdlib/dynthr_mod/dynthr_mod.c
+++ b/lib/libc/tests/stdlib/dynthr_mod/dynthr_mod.c
@@ -1,73 +1,74 @@
 /*
  * SPDX-License-Identifier: BSD-2-Clause
  *
  * Copyright (C) 2019 Andrew Gierth
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  *
  * Though this file is initially distributed under the 2-clause BSD license,
  * the author grants permission for its redistribution under alternative
  * licenses as set forth at <https://rhodiumtoad.github.io/RELICENSE.txt>.
  * This paragraph and the RELICENSE.txt file are not part of the license and
  * may be omitted in redistributions.
  */
 
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdarg.h>
 #include <string.h>
 #include <unistd.h>
 #include <pthread.h>
 
 void mod_main(int op);
 
 static pthread_t thr;
 
 static void *
 mod_thread(void *ptr __unused)
 {
 	char *volatile dummy;
 
 	dummy = malloc(500);
+	*dummy = 'a';
 	return (NULL);
 }
 
 void
 mod_main(int op)
 {
 	int rc;
 
 	switch (op) {
 	case 1:
 		rc = pthread_create(&thr, NULL, mod_thread, NULL);
 		if (rc != 0)
 			_exit(1);
 		break;
 	case 0:
 		pthread_join(thr, NULL);
 		break;
 	}
 }