diff --git a/eth2/types/src/attestation_duty.rs b/eth2/types/src/attestation_duty.rs
index 80d912a83f..299fdd44cf 100644
--- a/eth2/types/src/attestation_duty.rs
+++ b/eth2/types/src/attestation_duty.rs
@@ -6,4 +6,5 @@ pub struct AttestationDuty {
     pub slot: Slot,
     pub shard: Shard,
     pub committee_index: usize,
+    pub committee_len: usize,
 }
diff --git a/eth2/types/src/beacon_state/epoch_cache.rs b/eth2/types/src/beacon_state/epoch_cache.rs
index 32d9a643e9..62df902714 100644
--- a/eth2/types/src/beacon_state/epoch_cache.rs
+++ b/eth2/types/src/beacon_state/epoch_cache.rs
@@ -92,6 +92,7 @@ impl EpochCache {
                         slot,
                         shard,
                         committee_index: k,
+                        committee_len: crosslink_committee.committee.len(),
                     };
                     attestation_duties[*validator_index] = Some(attestation_duty)
                 }