diff --git a/specs/merge/validator.md b/specs/merge/validator.md
index bd3c9585e..678f7fe00 100644
--- a/specs/merge/validator.md
+++ b/specs/merge/validator.md
@@ -49,11 +49,10 @@ Please see related Beacon Chain doc before continuing and use them as a referenc
 ### `get_pow_block_at_terminal_total_difficulty`
 
 ```python
-def get_pow_block_at_terminal_total_difficulty(pow_chain: Sequence[PowBlock]) -> Optional[PowBlock]:
+def get_pow_block_at_terminal_total_difficulty(pow_chain: Dict[Hash32, PowBlock]) -> Optional[PowBlock]:
     # `pow_chain` abstractly represents all blocks in the PoW chain
     for block in pow_chain:
-        parent = get_pow_block(block.parent_hash)
-        assert parent is not None
+        parent = pow_chain[block.parent_hash]
         block_reached_ttd = block.total_difficulty >= TERMINAL_TOTAL_DIFFICULTY
         parent_reached_ttd = parent.total_difficulty >= TERMINAL_TOTAL_DIFFICULTY
         if block_reached_ttd and not parent_reached_ttd:
@@ -65,13 +64,13 @@ def get_pow_block_at_terminal_total_difficulty(pow_chain: Sequence[PowBlock]) ->
 ### `get_terminal_pow_block`
 
 ```python
-def get_terminal_pow_block(pow_chain: Sequence[PowBlock]) -> Optional[PowBlock]:
+def get_terminal_pow_block(pow_chain: Dict[Hash32, PowBlock]) -> Optional[PowBlock]:
     if TERMINAL_BLOCK_HASH != Hash32():
         # Terminal block hash override takes precedence over terminal total difficulty
-        pow_block_overrides = [block for block in pow_chain if block.block_hash == TERMINAL_BLOCK_HASH]
-        if not any(pow_block_overrides):
+        if TERMINAL_BLOCK_HASH in pow_chain:
+            return pow_chain[TERMINAL_BLOCK_HASH]
+        else:
             return None
-        return pow_block_overrides[0]
 
     return get_pow_block_at_terminal_total_difficulty(pow_chain)
 ```
@@ -133,14 +132,14 @@ To obtain an execution payload, a block proposer building a block on top of a `s
 
 1. Set `payload_id = prepare_execution_payload(state, pow_chain, finalized_block_hash, fee_recipient, execution_engine)`, where:
     * `state` is the state object after applying `process_slots(state, slot)` transition to the resulting state of the parent block processing
-    * `pow_chain` is a list that abstractly represents all blocks in the PoW chain
+    * `pow_chain` is a `Dict[Hash32, PowBlock]` dictionary that abstractly represents all blocks in the PoW chain with block hash as the dictionary key
     * `finalized_block_hash` is the hash of the latest finalized execution payload (`Hash32()` if none yet finalized)
     * `fee_recipient` is the value suggested to be used for the `coinbase` field of the execution payload
 
 
 ```python
 def prepare_execution_payload(state: BeaconState,
-                              pow_chain: Sequence[PowBlock],
+                              pow_chain: Dict[Hash32, PowBlock],
                               finalized_block_hash: Hash32,
                               fee_recipient: ExecutionAddress,
                               execution_engine: ExecutionEngine) -> Optional[PayloadId]:
diff --git a/tests/core/pyspec/eth2spec/test/phase0/block_processing/test_process_attestation.py b/tests/core/pyspec/eth2spec/test/phase0/block_processing/test_process_attestation.py
index c30320066..8d9e67998 100644
--- a/tests/core/pyspec/eth2spec/test/phase0/block_processing/test_process_attestation.py
+++ b/tests/core/pyspec/eth2spec/test/phase0/block_processing/test_process_attestation.py
@@ -275,10 +275,38 @@ def test_invalid_current_source_root(spec, state):
     state.previous_justified_checkpoint = spec.Checkpoint(epoch=3, root=b'\x01' * 32)
     state.current_justified_checkpoint = spec.Checkpoint(epoch=4, root=b'\x32' * 32)
 
-    attestation = get_valid_attestation(spec, state, slot=(spec.SLOTS_PER_EPOCH * 3) + 1)
+    next_slots(spec, state, spec.MIN_ATTESTATION_INCLUSION_DELAY)
+
+    attestation = get_valid_attestation(spec, state, slot=spec.SLOTS_PER_EPOCH * 5)
+
+    # Test logic sanity checks:
+    assert attestation.data.target.epoch == spec.get_current_epoch(state)
+    assert state.current_justified_checkpoint.root != state.previous_justified_checkpoint.root
+    assert attestation.data.source.root == state.current_justified_checkpoint.root
+
+    # Make attestation source root invalid: should be current justified, not previous one
+    attestation.data.source.root = state.previous_justified_checkpoint.root
+
+    sign_attestation(spec, state, attestation)
+
+    yield from run_attestation_processing(spec, state, attestation, False)
+
+
+@with_all_phases
+@spec_state_test
+def test_invalid_previous_source_root(spec, state):
+    next_slots(spec, state, spec.SLOTS_PER_EPOCH * 5)
+
+    state.finalized_checkpoint.epoch = 2
+
+    state.previous_justified_checkpoint = spec.Checkpoint(epoch=3, root=b'\x01' * 32)
+    state.current_justified_checkpoint = spec.Checkpoint(epoch=4, root=b'\x32' * 32)
+
+    attestation = get_valid_attestation(spec, state, slot=(spec.SLOTS_PER_EPOCH * 4) + 1)
     next_slots(spec, state, spec.MIN_ATTESTATION_INCLUSION_DELAY)
 
     # Test logic sanity checks:
+    assert attestation.data.target.epoch == spec.get_previous_epoch(state)
     assert state.current_justified_checkpoint.root != state.previous_justified_checkpoint.root
     assert attestation.data.source.root == state.previous_justified_checkpoint.root