diff --git a/specs/core/0_beacon-chain.md b/specs/core/0_beacon-chain.md
index f42b4cc13..1f500bc05 100644
--- a/specs/core/0_beacon-chain.md
+++ b/specs/core/0_beacon-chain.md
@@ -1553,7 +1553,9 @@ def process_final_updates(state: BeaconState) -> None:
     # Set active index root
     index_root_position = (next_epoch + ACTIVATION_EXIT_DELAY) % EPOCHS_PER_HISTORICAL_VECTOR
     state.active_index_roots[index_root_position] = hash_tree_root(
-        get_active_validator_indices(state, Epoch(next_epoch + ACTIVATION_EXIT_DELAY))
+        List[ValidatorIndex, VALIDATOR_REGISTRY_SIZE](
+            get_active_validator_indices(state, Epoch(next_epoch + ACTIVATION_EXIT_DELAY))
+        )
     )
     # Set total slashed balances
     state.slashed_balances[next_epoch % EPOCHS_PER_SLASHED_BALANCES_VECTOR] = (
diff --git a/test_libs/pyspec/eth2spec/test/helpers/custody.py b/test_libs/pyspec/eth2spec/test/helpers/custody.py
index bc70c9fa8..cd34ee4c8 100644
--- a/test_libs/pyspec/eth2spec/test/helpers/custody.py
+++ b/test_libs/pyspec/eth2spec/test/helpers/custody.py
@@ -4,7 +4,7 @@ from eth2spec.utils.bls import bls_sign
 
 def get_valid_early_derived_secret_reveal(spec, state, epoch=None):
     current_epoch = spec.get_current_epoch(state)
-    revealed_index = spec.get_active_validator_indices(state, current_epoch).last()
+    revealed_index = spec.get_active_validator_indices(state, current_epoch)[-1]
     masker_index = spec.get_active_validator_indices(state, current_epoch)[0]
 
     if epoch is None:
diff --git a/test_libs/pyspec/eth2spec/test/helpers/genesis.py b/test_libs/pyspec/eth2spec/test/helpers/genesis.py
index d1d818908..73738932a 100644
--- a/test_libs/pyspec/eth2spec/test/helpers/genesis.py
+++ b/test_libs/pyspec/eth2spec/test/helpers/genesis.py
@@ -40,7 +40,8 @@ def create_genesis_state(spec, num_validators):
             validator.activation_eligibility_epoch = spec.GENESIS_EPOCH
             validator.activation_epoch = spec.GENESIS_EPOCH
 
-    genesis_active_index_root = hash_tree_root(spec.get_active_validator_indices(state, spec.GENESIS_EPOCH))
+    genesis_active_index_root = hash_tree_root(List[ValidatorIndex, VALIDATOR_REGISTRY_SIZE](
+        spec.get_active_validator_indices(state, spec.GENESIS_EPOCH)))
     for index in range(spec.EPOCHS_PER_HISTORICAL_VECTOR):
         state.active_index_roots[index] = genesis_active_index_root
 
diff --git a/test_libs/pyspec/eth2spec/test/helpers/proposer_slashings.py b/test_libs/pyspec/eth2spec/test/helpers/proposer_slashings.py
index 7b6f71374..d5b7f7b7f 100644
--- a/test_libs/pyspec/eth2spec/test/helpers/proposer_slashings.py
+++ b/test_libs/pyspec/eth2spec/test/helpers/proposer_slashings.py
@@ -6,7 +6,7 @@ from eth2spec.test.helpers.keys import pubkey_to_privkey
 
 def get_valid_proposer_slashing(spec, state, signed_1=False, signed_2=False):
     current_epoch = spec.get_current_epoch(state)
-    validator_index = spec.get_active_validator_indices(state, current_epoch).last()
+    validator_index = spec.get_active_validator_indices(state, current_epoch)[-1]
     privkey = pubkey_to_privkey[state.validators[validator_index].pubkey]
     slot = state.slot
 
diff --git a/test_libs/pyspec/eth2spec/test/helpers/transfers.py b/test_libs/pyspec/eth2spec/test/helpers/transfers.py
index 215cd6fcb..acc6a35c5 100644
--- a/test_libs/pyspec/eth2spec/test/helpers/transfers.py
+++ b/test_libs/pyspec/eth2spec/test/helpers/transfers.py
@@ -9,7 +9,7 @@ def get_valid_transfer(spec, state, slot=None, sender_index=None, amount=None, f
         slot = state.slot
     current_epoch = spec.get_current_epoch(state)
     if sender_index is None:
-        sender_index = spec.get_active_validator_indices(state, current_epoch).last()
+        sender_index = spec.get_active_validator_indices(state, current_epoch)[-1]
     recipient_index = spec.get_active_validator_indices(state, current_epoch)[0]
     transfer_pubkey = pubkeys[-1]
     transfer_privkey = privkeys[-1]
diff --git a/test_libs/pyspec/eth2spec/test/phase_0/block_processing/test_process_transfer.py b/test_libs/pyspec/eth2spec/test/phase_0/block_processing/test_process_transfer.py
index 2bed94dc8..e9d282b3a 100644
--- a/test_libs/pyspec/eth2spec/test/phase_0/block_processing/test_process_transfer.py
+++ b/test_libs/pyspec/eth2spec/test/phase_0/block_processing/test_process_transfer.py
@@ -63,7 +63,7 @@ def test_success_withdrawable(spec, state):
 @with_all_phases
 @spec_state_test
 def test_success_active_above_max_effective(spec, state):
-    sender_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state)).last()
+    sender_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state))[-1]
     state.balances[sender_index] = spec.MAX_EFFECTIVE_BALANCE + 1
     transfer = get_valid_transfer(spec, state, sender_index=sender_index, amount=1, fee=0, signed=True)
 
@@ -73,7 +73,7 @@ def test_success_active_above_max_effective(spec, state):
 @with_all_phases
 @spec_state_test
 def test_success_active_above_max_effective_fee(spec, state):
-    sender_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state)).last()
+    sender_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state))[-1]
     state.balances[sender_index] = spec.MAX_EFFECTIVE_BALANCE + 1
     transfer = get_valid_transfer(spec, state, sender_index=sender_index, amount=0, fee=1, signed=True)
 
@@ -94,7 +94,7 @@ def test_invalid_signature(spec, state):
 @with_all_phases
 @spec_state_test
 def test_active_but_transfer_past_effective_balance(spec, state):
-    sender_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state)).last()
+    sender_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state))[-1]
     amount = spec.MAX_EFFECTIVE_BALANCE // 32
     state.balances[sender_index] = spec.MAX_EFFECTIVE_BALANCE
     transfer = get_valid_transfer(spec, state, sender_index=sender_index, amount=amount, fee=0, signed=True)
@@ -115,7 +115,7 @@ def test_incorrect_slot(spec, state):
 @with_all_phases
 @spec_state_test
 def test_insufficient_balance_for_fee(spec, state):
-    sender_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state)).last()
+    sender_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state))[-1]
     state.balances[sender_index] = spec.MAX_EFFECTIVE_BALANCE
     transfer = get_valid_transfer(spec, state, sender_index=sender_index, amount=0, fee=1, signed=True)
 
@@ -128,7 +128,7 @@ def test_insufficient_balance_for_fee(spec, state):
 @with_all_phases
 @spec_state_test
 def test_insufficient_balance(spec, state):
-    sender_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state)).last()
+    sender_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state))[-1]
     state.balances[sender_index] = spec.MAX_EFFECTIVE_BALANCE
     transfer = get_valid_transfer(spec, state, sender_index=sender_index, amount=1, fee=0, signed=True)
 
@@ -141,7 +141,7 @@ def test_insufficient_balance(spec, state):
 @with_all_phases
 @spec_state_test
 def test_no_dust_sender(spec, state):
-    sender_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state)).last()
+    sender_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state))[-1]
     balance = state.balances[sender_index]
     transfer = get_valid_transfer(
         spec,
@@ -161,7 +161,7 @@ def test_no_dust_sender(spec, state):
 @with_all_phases
 @spec_state_test
 def test_no_dust_recipient(spec, state):
-    sender_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state)).last()
+    sender_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state))[-1]
     state.balances[sender_index] = spec.MAX_EFFECTIVE_BALANCE + 1
     transfer = get_valid_transfer(spec, state, sender_index=sender_index, amount=1, fee=0, signed=True)
     state.balances[transfer.recipient] = 0
diff --git a/test_libs/pyspec/eth2spec/test/phase_0/block_processing/test_process_voluntary_exit.py b/test_libs/pyspec/eth2spec/test/phase_0/block_processing/test_process_voluntary_exit.py
index b9f7d6098..33cacc4e2 100644
--- a/test_libs/pyspec/eth2spec/test/phase_0/block_processing/test_process_voluntary_exit.py
+++ b/test_libs/pyspec/eth2spec/test/phase_0/block_processing/test_process_voluntary_exit.py
@@ -93,7 +93,7 @@ def test_success_exit_queue(spec, state):
             continue
 
     # exit an additional validator
-    validator_index = spec.get_active_validator_indices(state, current_epoch).last()
+    validator_index = spec.get_active_validator_indices(state, current_epoch)[-1]
     privkey = pubkey_to_privkey[state.validators[validator_index].pubkey]
     voluntary_exit = build_voluntary_exit(
         spec,
diff --git a/test_libs/pyspec/eth2spec/test/sanity/test_blocks.py b/test_libs/pyspec/eth2spec/test/sanity/test_blocks.py
index c86394a0b..f8590005e 100644
--- a/test_libs/pyspec/eth2spec/test/sanity/test_blocks.py
+++ b/test_libs/pyspec/eth2spec/test/sanity/test_blocks.py
@@ -269,7 +269,7 @@ def test_voluntary_exit(spec, state):
     validator_index = spec.get_active_validator_indices(
         state,
         spec.get_current_epoch(state)
-    ).last()
+    )[-1]
 
     # move state forward PERSISTENT_COMMITTEE_PERIOD epochs to allow for exit
     state.slot += spec.PERSISTENT_COMMITTEE_PERIOD * spec.SLOTS_PER_EPOCH
@@ -315,7 +315,7 @@ def test_voluntary_exit(spec, state):
     # overwrite default 0 to test
     # spec.MAX_TRANSFERS = 1
 
-    # sender_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state)).last()
+    # sender_index = spec.get_active_validator_indices(state, spec.get_current_epoch(state))[-1]
     # amount = get_balance(state, sender_index)
 
     # transfer = get_valid_transfer(spec, state, state.slot + 1, sender_index, amount, signed=True)
@@ -347,7 +347,7 @@ def test_voluntary_exit(spec, state):
 @spec_state_test
 def test_balance_driven_status_transitions(spec, state):
     current_epoch = spec.get_current_epoch(state)
-    validator_index = spec.get_active_validator_indices(state, current_epoch).last()
+    validator_index = spec.get_active_validator_indices(state, current_epoch)[-1]
 
     assert state.validators[validator_index].exit_epoch == spec.FAR_FUTURE_EPOCH