diff --git a/specs/core/0_beacon-chain.md b/specs/core/0_beacon-chain.md
index e594ab898..0725f5211 100644
--- a/specs/core/0_beacon-chain.md
+++ b/specs/core/0_beacon-chain.md
@@ -991,6 +991,7 @@ def convert_to_indexed(state: BeaconState, attestation: Attestation) -> IndexedA
     """
     attesting_indices = get_attesting_indices(state, attestation.data, attestation.aggregation_bitfield)
     custody_bit_1_indices = get_attesting_indices(state, attestation.data, attestation.custody_bitfield)
+    assert set(custody_bit_1_indices).issubset(attesting_indices)
     custody_bit_0_indices = [index for index in attesting_indices if index not in custody_bit_1_indices]
 
     return IndexedAttestation(
@@ -1722,6 +1723,10 @@ def process_attestation(state: BeaconState, attestation: Attestation) -> None:
     Process ``Attestation`` operation.
     """
     data = attestation.data
+
+    assert data.crosslink.shard < SHARD_COUNT
+    assert data.target_epoch in (get_previous_epoch(state), get_current_epoch(state))
+
     attestation_slot = get_attestation_data_slot(state, data)
     assert attestation_slot + MIN_ATTESTATION_INCLUSION_DELAY <= state.slot <= attestation_slot + SLOTS_PER_EPOCH
 
@@ -1732,7 +1737,6 @@ def process_attestation(state: BeaconState, attestation: Attestation) -> None:
         proposer_index=get_beacon_proposer_index(state),
     )
 
-    assert data.target_epoch in (get_previous_epoch(state), get_current_epoch(state))
     if data.target_epoch == get_current_epoch(state):
         ffg_data = (state.current_justified_epoch, state.current_justified_root, get_current_epoch(state))
         parent_crosslink = state.current_crosslinks[data.crosslink.shard]