diff --git a/plugins/core.c b/plugins/core.c
index 9d737d8278..a864275ae7 100644
--- a/plugins/core.c
+++ b/plugins/core.c
@@ -245,6 +245,7 @@ void qemu_plugin_vcpu_init_hook(CPUState *cpu)
 {
     bool success;
 
+    assert(cpu->cpu_index != UNASSIGNED_CPU_INDEX);
     qemu_rec_mutex_lock(&plugin.lock);
     plugin.num_vcpus = MAX(plugin.num_vcpus, cpu->cpu_index + 1);
     plugin_cpu_update__locked(&cpu->cpu_index, NULL, NULL);
@@ -263,6 +264,7 @@ void qemu_plugin_vcpu_exit_hook(CPUState *cpu)
 
     plugin_vcpu_cb__simple(cpu, QEMU_PLUGIN_EV_VCPU_EXIT);
 
+    assert(cpu->cpu_index != UNASSIGNED_CPU_INDEX);
     qemu_rec_mutex_lock(&plugin.lock);
     success = g_hash_table_remove(plugin.cpu_ht, &cpu->cpu_index);
     g_assert(success);