diff --git a/eth2/state_processing/src/common/convert_to_indexed.rs b/eth2/state_processing/src/common/convert_to_indexed.rs
index 5492252f7..1854d32d1 100644
--- a/eth2/state_processing/src/common/convert_to_indexed.rs
+++ b/eth2/state_processing/src/common/convert_to_indexed.rs
@@ -1,10 +1,10 @@
-use super::get_attesting_indices;
+use super::{get_attesting_indices, get_attesting_indices_unsorted};
 use itertools::{Either, Itertools};
 use types::*;
 
 /// Convert `attestation` to (almost) indexed-verifiable form.
 ///
-/// Spec v0.6.1
+/// Spec v0.6.3
 pub fn convert_to_indexed<T: EthSpec>(
     state: &BeaconState<T>,
     attestation: &Attestation,
@@ -12,6 +12,10 @@ pub fn convert_to_indexed<T: EthSpec>(
     let attesting_indices =
         get_attesting_indices(state, &attestation.data, &attestation.aggregation_bitfield)?;
 
+    // We verify the custody bitfield by calling `get_attesting_indices_unsorted` and throwing
+    // away the result. This avoids double-sorting - the partition below takes care of the ordering.
+    get_attesting_indices_unsorted(state, &attestation.data, &attestation.custody_bitfield)?;
+
     let (custody_bit_0_indices, custody_bit_1_indices) =
         attesting_indices.into_iter().enumerate().partition_map(
             |(committee_idx, validator_idx)| match attestation.custody_bitfield.get(committee_idx) {