diff --git a/sync/optimistic.md b/sync/optimistic.md
index cf9bc2d54..e36378119 100644
--- a/sync/optimistic.md
+++ b/sync/optimistic.md
@@ -278,6 +278,7 @@ When information about an optimistic block is requested, the consensus engine:
 
 When `is_optimistic(head) == True`, the consensus engine:
 
+- MUST NOT return `head`.
 - MAY substitute the head block with `latest_valid_ancestor(block)`.
 - MAY return syncing.