diff --git a/scripts/build_spec.py b/scripts/build_spec.py
index 237c24384..d67c0e5c6 100644
--- a/scripts/build_spec.py
+++ b/scripts/build_spec.py
@@ -37,8 +37,7 @@ from eth2spec.utils.bls import (
 from eth2spec.utils.hash_function import hash
 '''
 PHASE1_IMPORTS = '''from typing import (
-    Any, Callable, Dict, Optional, Set, Sequence, Tuple,
-    List as TypingList
+    Any, Callable, Dict, Optional, Set, Sequence, MutableSequence, Tuple,
 )
 
 from dataclasses import (
diff --git a/specs/core/1_custody-game.md b/specs/core/1_custody-game.md
index 24e9a19e2..07f6ec698 100644
--- a/specs/core/1_custody-game.md
+++ b/specs/core/1_custody-game.md
@@ -318,7 +318,7 @@ def get_validators_custody_reveal_period(state: BeaconState,
 ### `replace_empty_or_append`
 
 ```python
-def replace_empty_or_append(list: TypingList[Any], new_element: Any) -> int:
+def replace_empty_or_append(list: MutableSequence[Any], new_element: Any) -> int:
     for i in range(len(list)):
         if is_empty(list[i]):
             list[i] = new_element
diff --git a/specs/core/1_shard-data-chains.md b/specs/core/1_shard-data-chains.md
index dd48a842a..d1c86eca2 100644
--- a/specs/core/1_shard-data-chains.md
+++ b/specs/core/1_shard-data-chains.md
@@ -252,9 +252,9 @@ def compute_crosslink_data_root(blocks: Sequence[ShardBlock]) -> Bytes32:
     def is_power_of_two(value: int) -> bool:
         return (value > 0) and (value & (value - 1) == 0)
 
-    def pad_to_power_of_2(values: TypingList[bytes]) -> TypingList[bytes]:
+    def pad_to_power_of_2(values: MutableSequence[bytes]) -> Sequence[bytes]:
         while not is_power_of_two(len(values)):
-            values += [b'\x00' * BYTES_PER_SHARD_BLOCK_BODY]
+            values.append(b'\x00' * BYTES_PER_SHARD_BLOCK_BODY)
         return values
 
     def hash_tree_root_of_bytes(data: bytes) -> bytes:
@@ -264,6 +264,8 @@ def compute_crosslink_data_root(blocks: Sequence[ShardBlock]) -> Bytes32:
         return data + b'\x00' * (length - len(data))
 
     return hash(
+        # TODO untested code.
+        #  Need to either pass a typed list to hash-tree-root, or merkleize_chunks(values, pad_to=2**x)
         hash_tree_root(pad_to_power_of_2([
             hash_tree_root_of_bytes(
                 zpad(serialize(get_shard_header(block)), BYTES_PER_SHARD_BLOCK_BODY)