diff --git a/scripts/build_spec.py b/scripts/build_spec.py
index 0a5171e8f..83f9a2145 100644
--- a/scripts/build_spec.py
+++ b/scripts/build_spec.py
@@ -37,7 +37,7 @@ from eth2spec.utils.bls import (
 from eth2spec.utils.hash_function import hash
 '''
 PHASE1_IMPORTS = '''from typing import (
-    Any, Dict, Optional, Set, Sequence, MutableSequence, Tuple, Union,
+    Any, Dict, Optional, Set, Sequence, MutableSequence, NewType, Tuple, Union,
 )
 from math import (
     log2,
@@ -70,6 +70,7 @@ from eth2spec.utils.hash_function import hash
 
 
 SSZVariableName = str
+GeneralizedIndex = NewType('GeneralizedIndex', int)
 '''
 SUNDRY_CONSTANTS_FUNCTIONS = '''
 def ceillog2(x: uint64) -> int:
diff --git a/specs/light_client/merkle_proofs.md b/specs/light_client/merkle_proofs.md
index d7f0ab382..ce7dc647c 100644
--- a/specs/light_client/merkle_proofs.md
+++ b/specs/light_client/merkle_proofs.md
@@ -7,7 +7,6 @@
 
 - [Merkle proof formats](#merkle-proof-formats)
     - [Table of contents](#table-of-contents)
-    - [Custom types](#custom-types)
     - [Helper functions](#helper-functions)
     - [Generalized Merkle tree index](#generalized-merkle-tree-index)
     - [SSZ object to index](#ssz-object-to-index)
@@ -22,13 +21,6 @@
 
 <!-- /TOC -->
 
-## Custom types
-
-We define the following Python custom types for type hinting and readability:
-
-| - | - | - |
-| `GeneralizedIndex` | `uint64` | the index of a node in a binary Merkle tree |
-
 ## Helper functions
 
 ```python
@@ -75,6 +67,8 @@ def merkle_tree(leaves: Sequence[Hash]) -> Sequence[Hash]:
     return o
 ```
 
+We define a custom type `GeneralizedIndex` as a Python integer type in this document. It can be represented as a Bitvector/Bitlist object as well.
+
 We will define Merkle proofs in terms of generalized indices.
 
 ## SSZ object to index
@@ -175,7 +169,7 @@ def get_generalized_index(typ: SSZType, path: Sequence[Union[int, SSZVariableNam
         else:
             pos, _, _ = get_item_position(typ, p)
             base_index = (GeneralizedIndex(2) if issubclass(typ, (List, Bytes)) else GeneralizedIndex(1))
-            root = root * base_index * get_next_power_of_two(chunk_count(typ)) + pos
+            root = GeneralizedIndex(root * base_index * get_next_power_of_two(chunk_count(typ)) + pos)
             typ = get_elem_type(typ, p)
     return root
 ```
@@ -280,8 +274,8 @@ def get_helper_indices(indices: Sequence[GeneralizedIndex]) -> Sequence[Generali
     return sorted([
         x for x in all_indices if (
             not (
-                generalized_index_child(x, GeneralizedIndex(0)) in all_indices and
-                generalized_index_child(x, GeneralizedIndex(1)) in all_indices
+                generalized_index_child(x, False) in all_indices and
+                generalized_index_child(x, True) in all_indices
             ) and not (x in indices)
         )
     ], reverse=True)