diff --git a/specs/casper_sharding_v2.1.md b/specs/casper_sharding_v2.1.md
index 1359b8153..3254050ac 100644
--- a/specs/casper_sharding_v2.1.md
+++ b/specs/casper_sharding_v2.1.md
@@ -475,7 +475,7 @@ For each one of these attestations:
 * Verify that `len(attester_bitfield) == ceil_div8(len(attestation_indices))`, where `ceil_div8 = (x + 7) // 8`. Verify that bits `len(attestation_indices)....` and higher, if present (i.e. `len(attestation_indices)` is not a multiple of 8), are all zero
 * Derive a group public key by adding the public keys of all of the attesters in `attestation_indices` for whom the corresponding bit in `attester_bitfield` (the ith bit is `(attester_bitfield[i // 8] >> (7 - (i %8))) % 2`) equals 1
 * Let `version = pre_fork_version if slot < fork_slot_number else post_fork_version`.
-* Verify that `aggregate_sig` verifies using the group pubkey generated and the serialized form of `AttestationSignedData(version, slot, parent_hashes, shard_id, shard_block_hash, crystallized_state.last_justified_slot)` as the message.
+* Verify that `aggregate_sig` verifies using the group pubkey generated and the serialized form of `AttestationSignedData(version, slot, parent_hashes, shard_id, shard_block_hash, attestation.last_justified_slot)` as the message.
 
 Extend the list of `AttestationRecord` objects in the `active_state` with those included in the block, ordering the new additions in the same order as they came in the block. Similarly extend the list of `SpecialObject` objects in the `active_state` with those included in the block.