--- a/sys/kern/subr_cpu_topology.c
+++ b/sys/kern/subr_cpu_topology.c
@@ -719,11 +719,19 @@ static
 void
 sbuf_print_cpuset(struct sbuf *sb, cpumask_t *mask)
 {
 	int i;
 	int b = -1;
 	int e = -1;
 	int more = 0;
 
 	sbuf_printf(sb, "cpus(");
-	CPUSET_FOREACH(i, *mask) {
+	/*
+	 * CPUSET_FOREACH is bounded by ncpus, but this helper runs
+	 * during SI_BOOT2_CPU_TOPOLOGY, i.e. before the APs are
+	 * started and before ncpus is set to naps + 1 (ncpus == 1
+	 * there).  Iterate the full mask width instead so the
+	 * boot-time renderings of hw.cpu_topology.members and the
+	 * per-cpu physical_siblings/core_siblings strings are
+	 * complete.  CPUMASK_TESTBIT masks the bit index itself.
+	 */
+	for (i = 0; i < MAXCPU; ++i) {
+		if (!CPUMASK_TESTBIT(*mask, i))
+			continue;
 		if (b < 0) {
 			b = i;
 			e = b + 1;
 			continue;
```