diff --git a/consensus/types/src/proposer_preparation_data.rs b/consensus/types/src/proposer_preparation_data.rs
index f2ea96711..6179828a9 100644
--- a/consensus/types/src/proposer_preparation_data.rs
+++ b/consensus/types/src/proposer_preparation_data.rs
@@ -6,6 +6,7 @@ use serde::{Deserialize, Serialize};
 #[derive(PartialEq, Debug, Serialize, Deserialize, Clone)]
 pub struct ProposerPreparationData {
     /// The validators index.
+    #[serde(with = "eth2_serde_utils::quoted_u64")]
     pub validator_index: u64,
     /// The fee-recipient address.
     pub fee_recipient: Address,