diff --git a/specs/phase0/fork-choice.md b/specs/phase0/fork-choice.md
index 64d48d4b5..39d9bd402 100644
--- a/specs/phase0/fork-choice.md
+++ b/specs/phase0/fork-choice.md
@@ -279,9 +279,13 @@ def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
     assert get_current_slot(store) >= block.slot
     # Add new block to the store
     store.blocks[hash_tree_root(block)] = block
-    # Check block is a descendant of the finalized block
+
+    # Check that block is later than the finalized epoch slot (optimization to reduce calls to get_ancestor)
     finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
+    assert block.slot > finalized_slot
+    # Check block is a descendant of the finalized block
     assert get_ancestor(store, hash_tree_root(block), finalized_slot) == store.finalized_checkpoint.root
+
     # Check the block is valid and compute the post-state
     state = state_transition(pre_state, signed_block, True)
     # Add new state for this block to the store