diff --git a/src/vlib/main.h b/src/vlib/main.h
index 598c3ba..d6b2d1f 100644
--- a/src/vlib/main.h
+++ b/src/vlib/main.h
@@ -290,6 +290,13 @@
 always_inline f64
 vlib_time_now (vlib_main_t * vm)
 {
+#if CLIB_DEBUG > 0
+  extern __thread uword __os_thread_index;
+#endif
+  /*
+   * Make sure folks don't pass &vlib_global_main from a worker thread.
+   */
+  ASSERT (vm->thread_index == __os_thread_index);
   return clib_time_now (&vm->clib_time) + vm->time_offset;
 }