diff --git a/eth2/types/src/beacon_state/epoch_cache.rs b/eth2/types/src/beacon_state/epoch_cache.rs
index bbc991646..e6bacd351 100644
--- a/eth2/types/src/beacon_state/epoch_cache.rs
+++ b/eth2/types/src/beacon_state/epoch_cache.rs
@@ -33,12 +33,13 @@ impl EpochCache {
     ) -> Result<EpochCache, Error> {
         let mut epoch_committees: Vec<CrosslinkCommittees> =
             Vec::with_capacity(spec.slots_per_epoch as usize);
-        let mut attestation_duty_map: AttestationDutyMap = HashMap::new();
         let mut shard_committee_index_map: ShardCommitteeIndexMap = HashMap::new();
 
         let shuffling =
             state.get_shuffling_for_slot(epoch.start_slot(spec.slots_per_epoch), false, spec)?;
 
+        let mut attestation_duty_map: AttestationDutyMap = HashMap::with_capacity(shuffling.len());
+
         for (epoch_committeess_index, slot) in epoch.slot_iter(spec.slots_per_epoch).enumerate() {
             let slot_committees = state.calculate_crosslink_committees_at_slot(
                 slot,