diff --git a/coregrind/pub_core_threadstate.h b/coregrind/pub_core_threadstate.h
index bcba985c5..e7bde55c5 100644
--- a/coregrind/pub_core_threadstate.h
+++ b/coregrind/pub_core_threadstate.h
@@ -114,7 +114,9 @@ typedef
    ThreadArchState;
 
 
-/* OS-specific thread state */
+/* OS-specific thread state.  IMPORTANT: if you add fields to this,
+   you _must_ add code to os_state_clear() to initialise those
+   fields. */
 typedef
    struct {
       /* who we are */