diff --git a/cpu-common.c b/cpu-common.c
index 54e63b3f77965a4cb5f4ba906a9c5e0292e63712..a949ad7ca3cf5a526efed41c089d0646a5b10faa 100644
--- a/cpu-common.c
+++ b/cpu-common.c
@@ -234,6 +234,9 @@ 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;