diff --git a/common/eth2/src/lib.rs b/common/eth2/src/lib.rs
index bdad67286..153667d7e 100644
--- a/common/eth2/src/lib.rs
+++ b/common/eth2/src/lib.rs
@@ -1256,8 +1256,12 @@ impl BeaconNodeHttpClient {
             .push("attester")
             .push(&epoch.to_string());
 
-        self.post_with_timeout_and_response(path, &indices, self.timeouts.attester_duties)
-            .await
+        self.post_with_timeout_and_response(
+            path,
+            &ValidatorIndexDataRef(indices),
+            self.timeouts.attester_duties,
+        )
+        .await
     }
 
     /// `POST validator/aggregate_and_proofs`
@@ -1356,8 +1360,12 @@ impl BeaconNodeHttpClient {
             .push("sync")
             .push(&epoch.to_string());
 
-        self.post_with_timeout_and_response(path, &indices, self.timeouts.sync_duties)
-            .await
+        self.post_with_timeout_and_response(
+            path,
+            &ValidatorIndexDataRef(indices),
+            self.timeouts.sync_duties,
+        )
+        .await
     }
 }
 
diff --git a/common/eth2/src/types.rs b/common/eth2/src/types.rs
index 169a8de59..a761b9ed1 100644
--- a/common/eth2/src/types.rs
+++ b/common/eth2/src/types.rs
@@ -591,6 +591,13 @@ pub struct ValidatorBalancesQuery {
 #[serde(transparent)]
 pub struct ValidatorIndexData(#[serde(with = "eth2_serde_utils::quoted_u64_vec")] pub Vec<u64>);
 
+/// Borrowed variant of `ValidatorIndexData`, for serializing/sending.
+#[derive(Clone, Copy, Serialize)]
+#[serde(transparent)]
+pub struct ValidatorIndexDataRef<'a>(
+    #[serde(serialize_with = "eth2_serde_utils::quoted_u64_vec::serialize")] pub &'a [u64],
+);
+
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct AttesterData {
     pub pubkey: PublicKeyBytes,