diff --git a/specs/core/0_beacon-chain.md b/specs/core/0_beacon-chain.md
index f0169f1d2..c17815ee8 100644
--- a/specs/core/0_beacon-chain.md
+++ b/specs/core/0_beacon-chain.md
@@ -1654,6 +1654,9 @@ def process_attestation(state: BeaconState, attestation: Attestation) -> None:
     attestation_slot = get_attestation_data_slot(state, data)
     assert attestation_slot + MIN_ATTESTATION_INCLUSION_DELAY <= state.slot <= attestation_slot + SLOTS_PER_EPOCH
 
+    assert len(attestation.aggregation_bits) == len(committee)
+    assert len(attestation.custody_bits) == len(committee)
+
     pending_attestation = PendingAttestation(
         data=data,
         aggregation_bits=attestation.aggregation_bits,