diff --git a/specs/altair/validator.md b/specs/altair/validator.md
index 3b3362b22..7469552fb 100644
--- a/specs/altair/validator.md
+++ b/specs/altair/validator.md
@@ -282,7 +282,7 @@ def get_sync_committee_signature(state: BeaconState,
     signing_root = compute_signing_root(block_root, domain)
     signature = bls.Sign(privkey, signing_root)
 
-    return SyncCommitteeSignature(slot=state.slot, validator_index=validator_index, signature=signature)
+    return SyncCommitteeSignature(slot=state.slot, beacon_block_root=block_root, validator_index=validator_index, signature=signature)
 ```
 
 ##### Broadcast sync committee signature