diff --git a/src/emu/nosv/breakdown.c b/src/emu/nosv/breakdown.c
index 2a3d81b..da23448 100644
--- a/src/emu/nosv/breakdown.c
+++ b/src/emu/nosv/breakdown.c
@@ -1,4 +1,4 @@
-/* Copyright (c) 2023 Barcelona Supercomputing Center (BSC)
+/* Copyright (c) 2023-2024 Barcelona Supercomputing Center (BSC)
  * SPDX-License-Identifier: GPL-3.0-or-later */
 
 #include "breakdown.h"
@@ -181,8 +181,8 @@ connect_cpu(struct bay *bay, struct nosv_cpu *mcpu)
 		return -1;
 	}
 
-	/* TODO what do we emit on null? */
-	mux_set_default(&bcpu->mux0, value_int64(666));
+	/* Emit unknown subsystem on NULL */
+	mux_set_default(&bcpu->mux0, value_int64(ST_UNKNOWN_SS));
 
 	/* Connect mux 1 using idle as select */
 	if (mux_init(&bcpu->mux1, bay, idle, tri, select_idle, 2) != 0) {
diff --git a/src/emu/nosv/nosv_priv.h b/src/emu/nosv/nosv_priv.h
index 29edd61..48d4de3 100644
--- a/src/emu/nosv/nosv_priv.h
+++ b/src/emu/nosv/nosv_priv.h
@@ -24,6 +24,7 @@ enum nosv_chan {
 };
 
 enum nosv_ss_values {
+	ST_UNKNOWN_SS = 2,
 	ST_SCHED_HUNGRY = 6,
 	ST_SCHED_SERVING,
 	ST_SCHED_SUBMITTING,
diff --git a/src/emu/nosv/setup.c b/src/emu/nosv/setup.c
index 5539e5c..4618cef 100644
--- a/src/emu/nosv/setup.c
+++ b/src/emu/nosv/setup.c
@@ -141,6 +141,7 @@ static const char *pcf_prefix[CH_MAX] = {
 };
 
 static const struct pcf_value_label nosv_ss_values[] = {
+	{ ST_UNKNOWN_SS,       "Unknown subsystem" },
 	{ ST_SCHED_HUNGRY,     "Scheduler: Hungry" },
 	{ ST_SCHED_SERVING,    "Scheduler: Serving" },
 	{ ST_SCHED_SUBMITTING, "Scheduler: Submitting" },