diff --git a/specs/core/0_beacon-chain.md b/specs/core/0_beacon-chain.md
index 86df18898..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(