diff --git a/cpu-common.c b/cpu-common.c
index 6b262233a3..0d607bbe49 100644
--- a/cpu-common.c
+++ b/cpu-common.c
@@ -194,6 +194,9 @@ void start_exclusive(void)
     CPUState *other_cpu;
     int running_cpus;
 
+    /* Ensure we are not running, or start_exclusive will be blocked. */
+    g_assert(!current_cpu->running);
+
     if (current_cpu->exclusive_context_count) {
         current_cpu->exclusive_context_count++;
         return;