diff --git a/qemu-timer.c b/qemu-timer.c
index 9be1a41..00a5d35 100644
--- a/qemu-timer.c
+++ b/qemu-timer.c
@@ -126,6 +126,9 @@
 {
     QEMUClock *clock = qemu_clock_ptr(type);
 
+    /* Assert that the clock of type TYPE has not been initialized yet. */
+    assert(main_loop_tlg.tl[type] == NULL);
+
     clock->type = type;
     clock->enabled = true;
     clock->last = INT64_MIN;