diff --git a/specs/core/1_beacon-chain-misc.md b/specs/core/1_beacon-chain-misc.md
index 21b754156..bd7130f2c 100644
--- a/specs/core/1_beacon-chain-misc.md
+++ b/specs/core/1_beacon-chain-misc.md
@@ -129,7 +129,7 @@ def compute_historical_state_generalized_index(earlier: ShardSlot, later: ShardS
     for i in range(63, -1, -1):
         if (later - 1) & 2**i > (earlier - 1) & 2**i:
             later = later - ((later - 1) % 2**i) - 1
-            o = concat_generalized_indices(o, get_generalized_index(ShardState, ['history_acc', i]))
+            o = concat_generalized_indices(o, GeneralizedIndex(get_generalized_index(ShardState, ['history_acc', i])))
     return o
 ```
 
@@ -158,9 +158,9 @@ def process_shard_receipt_proof(state: BeaconState, receipt_proof: ShardReceiptP
     first_slot_in_last_crosslink = state.current_crosslinks[receipt_proof.shard].start_epoch * SLOTS_PER_EPOCH
     gindex = concat_generalized_indices(
         get_generalized_index_of_crosslink_header(0),
-        get_generalized_index(ShardBlockHeader, 'state_root'),
+        GeneralizedIndex(get_generalized_index(ShardBlockHeader, 'state_root')),
         compute_historical_state_generalized_index(receipt_slot, first_slot_in_last_crosslink),
-        get_generalized_index(ShardState, 'receipt_root')
+        GeneralizedIndex(get_generalized_index(ShardState, 'receipt_root'))
     )
     assert verify_merkle_proof(
         leaf=hash_tree_root(receipt_proof.receipt),
diff --git a/specs/light_client/merkle_proofs.md b/specs/light_client/merkle_proofs.md
index 344f365b0..bbd03d379 100644
--- a/specs/light_client/merkle_proofs.md
+++ b/specs/light_client/merkle_proofs.md
@@ -152,7 +152,7 @@ def get_item_position(typ: SSZType, index_or_variable_name: Union[int, SSZVariab
 ```
 
 ```python
-def get_generalized_index(typ: SSZType, path: Sequence[Union[int, SSZVariableName]]) -> Optional[GeneralizedIndex]:
+def get_generalized_index(typ: SSZType, path: Sequence[Union[int, SSZVariableName]]) -> GeneralizedIndex:
     """
     Converts a path (eg. `[7, "foo", 3]` for `x[7].foo[3]`, `[12, "bar", "__len__"]` for
     `len(x[12].bar)`) into the generalized index representing its position in the Merkle tree.
@@ -162,10 +162,8 @@ def get_generalized_index(typ: SSZType, path: Sequence[Union[int, SSZVariableNam
         assert not issubclass(typ, BasicValue)  # If we descend to a basic type, the path cannot continue further
         if p == '__len__':
             typ = uint64
-            if issubclass(typ, (List, Bytes)):
-                root = GeneralizedIndex(root * 2 + 1)
-            else:
-                return None
+            assert issubclass(typ, (List, Bytes))
+            root = GeneralizedIndex(root * 2 + 1)
         else:
             pos, _, _ = get_item_position(typ, p)
             base_index = (GeneralizedIndex(2) if issubclass(typ, (List, Bytes)) else GeneralizedIndex(1))
diff --git a/test_libs/pyspec/eth2spec/test/merkle_proofs/test_merkle_proofs.py b/test_libs/pyspec/eth2spec/test/merkle_proofs/test_merkle_proofs.py
index 91c861de3..62a2f6379 100644
--- a/test_libs/pyspec/eth2spec/test/merkle_proofs/test_merkle_proofs.py
+++ b/test_libs/pyspec/eth2spec/test/merkle_proofs/test_merkle_proofs.py
@@ -1,10 +1,10 @@
-
 import re
 from eth_utils import (
     to_tuple,
 )
 
 from eth2spec.test.context import (
+    expect_assertion_error,
     spec_state_test,
     with_all_phases_except,
 )
@@ -89,10 +89,14 @@ generalized_index_cases = [
 @spec_state_test
 def test_get_generalized_index(spec, state):
     for typ, path, generalized_index in generalized_index_cases:
-        assert spec.get_generalized_index(
-            typ=typ,
-            path=path,
-        ) == generalized_index
+        if generalized_index is not None:
+            assert spec.get_generalized_index(
+                typ=typ,
+                path=path,
+            ) == generalized_index
+        else:
+            expect_assertion_error(lambda: spec.get_generalized_index(typ=typ, path=path))
+
         yield 'typ', typ
         yield 'path', path
         yield 'generalized_index', generalized_index