diff --git a/specs/phase1/shard-fork-choice.md b/specs/phase1/shard-fork-choice.md
index fb98893ac..b60edd948 100644
--- a/specs/phase1/shard-fork-choice.md
+++ b/specs/phase1/shard-fork-choice.md
@@ -127,7 +127,6 @@ def on_shard_block(store: Store, shard_store: ShardStore, signed_shard_block: Si
 
     # Check beacon parent exists
     assert shard_block.beacon_parent_root in store.block_states
-    beacon_state = store.block_states[shard_block.beacon_parent_root]
 
     # Check that block is later than the finalized shard state slot (optimization to reduce calls to get_ancestor)
     finalized_beacon_state = store.block_states[store.finalized_checkpoint.root]
@@ -144,9 +143,11 @@ def on_shard_block(store: Store, shard_store: ShardStore, signed_shard_block: Si
     shard_store.blocks[hash_tree_root(shard_block)] = shard_block
 
     # Check the block is valid and compute the post-state
-    verify_shard_block_message(beacon_state, pre_shard_state, shard_block, shard_block.slot, shard)
-    verify_shard_block_signature(beacon_state, signed_shard_block)
-    post_state = get_post_shard_state(beacon_state, pre_shard_state, shard_block)
+    beacon_head_root = get_head(store)
+    beacon_head_state = store.block_states[beacon_head_root]
+    assert verify_shard_block_message(beacon_head_state, pre_shard_state, shard_block, shard_block.slot, shard)
+    assert verify_shard_block_signature(beacon_head_state, signed_shard_block)
+    post_state = get_post_shard_state(beacon_head_state, pre_shard_state, shard_block)
     # Add new state for this block to the store
     shard_store.block_states[hash_tree_root(shard_block)] = post_state
 ```
diff --git a/specs/phase1/shard-transition.md b/specs/phase1/shard-transition.md
index 5e8616568..6c4f652d2 100644
--- a/specs/phase1/shard-transition.md
+++ b/specs/phase1/shard-transition.md
@@ -50,6 +50,9 @@ def verify_shard_block_message(beacon_state: BeaconState,
                                shard: Shard) -> bool:
     assert block.shard_parent_root == shard_state.latest_block_root
     assert block.slot == slot
+    next_slot = Slot(beacon_state.slot + 1)
+    offset_slots = compute_offset_slots(get_latest_slot_for_shard(beacon_state, shard), next_slot)
+    assert slot in offset_slots
     assert block.shard == shard
     assert block.proposer_index == get_shard_proposer_index(beacon_state, slot, shard)
     assert 0 < len(block.body) <= MAX_SHARD_BLOCK_SIZE