diff --git a/src/include/cbmem.h b/src/include/cbmem.h
index 2e947a1973..524bfb5748 100644
--- a/src/include/cbmem.h
+++ b/src/include/cbmem.h
@@ -80,7 +80,10 @@ void cbmem_initialize_empty_id_size(u32 id, u64 size);
 
 /* Return the top address for dynamic cbmem. The address returned needs to
  * be consistent across romstage and ramstage, and it is required to be
- * below 4GiB. */
+ * below 4GiB.
+ * Board or chipset should return NULL if any interface that might rely on cbmem
+ * (e.g. cbfs, vboot) is used before the cbmem backing store has been
+ * initialized. */
 void *cbmem_top(void);
 
 /* Add a cbmem entry of a given size and id. These return NULL on failure. The