diff --git a/include/vm.h b/include/vm.h
index 8496297..c414ae2 100644
--- a/include/vm.h
+++ b/include/vm.h
@@ -110,8 +110,8 @@ void init_libraries(void);
 void exit_libraries(void);
 
 void vm_stack_trace(void);
-noreturn void _vm_error(const char *fmt, ...);
-noreturn void vm_exit(int code);
+_Noreturn void _vm_error(const char *fmt, ...);
+_Noreturn void vm_exit(int code);
 
 #define VM_ERROR(fmt, ...) \
 	_vm_error("*ERROR*(%s:%s:%d): " fmt "\n", __FILE__, __func__, __LINE__, ##__VA_ARGS__)
diff --git a/src/vm.c b/src/vm.c
index 79c127b..c5854e5 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -408,7 +408,7 @@ static const SDL_MessageBoxButtonData buttons[] = {
 	{ SDL_MESSAGEBOX_BUTTON_ESCAPEKEY_DEFAULT, 0, "Cancel" },
 };
 
-static noreturn void vm_reset(void);
+static _Noreturn void vm_reset(void);
 
 static struct string *get_func_stack_name(int index)
 {
@@ -2282,7 +2282,7 @@ static void vm_free(void)
 
 static jmp_buf reset_buf;
 
-static noreturn void vm_reset(void)
+static _Noreturn void vm_reset(void)
 {
 	vm_free();
 	longjmp(reset_buf, 1);
@@ -2356,7 +2356,7 @@ void vm_stack_trace(void)
 	}
 }
 
-noreturn void _vm_error(const char *fmt, ...)
+_Noreturn void _vm_error(const char *fmt, ...)
 {
 	va_list ap;
 	va_start(ap, fmt);
@@ -2377,7 +2377,7 @@ int vm_time(void)
 	return (ts.tv_sec * 1000) + (ts.tv_nsec / 1000000);
 }
 
-noreturn void vm_exit(int code)
+_Noreturn void vm_exit(int code)
 {
 	vm_free();
 #ifdef DEBUG_HEAP
diff --git a/subprojects/libsys4 b/subprojects/libsys4
index 958e1b6..499b505 160000
--- a/subprojects/libsys4
+++ b/subprojects/libsys4
@@ -1 +1 @@
-Subproject commit 958e1b676243a9b5b9225966e98c62b920ed5569
+Subproject commit 499b505ad9a99d85cd1b2747d09364c4e2f5de2b