diff --git a/test/zdtm/static/sigaltstack.c b/test/zdtm/static/sigaltstack.c
index be5ddf792..5f0428d78 100644
--- a/test/zdtm/static/sigaltstack.c
+++ b/test/zdtm/static/sigaltstack.c
@@ -64,7 +64,7 @@ void thread_sigaction(int signo, siginfo_t *info, void *context)
 static void *thread_func(void *arg)
 {
 	sas_state[SAS_THRD_OLD] = (stack_t) {
-		.ss_size	= SIGSTKSZ,
+		.ss_size	= sizeof(stack_thread) - 8,
 		.ss_sp		= stack_thread,
 		.ss_flags	= SS_ONSTACK,
 	};
@@ -106,7 +106,7 @@ int main(int argc, char *argv[])
 	pthread_t thread;
 
 	sas_state[SAS_MAIN_OLD] = (stack_t) {
-		.ss_size	= SIGSTKSZ,
+		.ss_size	= sizeof(stack_main) - 8,
 		.ss_sp		= stack_main,
 		.ss_flags	= SS_ONSTACK,
 	};