diff --git a/xen/common/sched/core.c b/xen/common/sched/core.c
index 2e43f8029f..de5a6b1a57 100644
--- a/xen/common/sched/core.c
+++ b/xen/common/sched/core.c
@@ -308,17 +308,26 @@ void vcpu_runstate_get(const struct vcpu *v,
 {
     spinlock_t *lock;
     s_time_t delta;
+    struct sched_unit *unit;
 
     rcu_read_lock(&sched_res_rculock);
 
-    lock = likely(v == current) ? NULL : unit_schedule_lock_irq(v->sched_unit);
+    /*
+     * Be careful in case of an idle vcpu: the assignment to a unit might
+     * change even with the scheduling lock held, so be sure to use the
+     * correct unit for locking in order to avoid triggering an ASSERT() in
+     * the unlock function.
+     */
+    unit = is_idle_vcpu(v) ? get_sched_res(v->processor)->sched_unit_idle
+                           : v->sched_unit;
+    lock = likely(v == current) ? NULL : unit_schedule_lock_irq(unit);
     memcpy(runstate, &v->runstate, sizeof(*runstate));
     delta = NOW() - runstate->state_entry_time;
     if ( delta > 0 )
         runstate->time[runstate->state] += delta;
 
     if ( unlikely(lock != NULL) )
-        unit_schedule_unlock_irq(lock, v->sched_unit);
+        unit_schedule_unlock_irq(lock, unit);
 
     rcu_read_unlock(&sched_res_rculock);
 }