diff --git a/packages/rln/src/contract/rln_contract.ts b/packages/rln/src/contract/rln_contract.ts
index e3b9dfef41..3943208c5d 100644
--- a/packages/rln/src/contract/rln_contract.ts
+++ b/packages/rln/src/contract/rln_contract.ts
@@ -476,7 +476,7 @@ export class RLNContract {
         identity,
         membership: {
           address,
-          treeIndex: membershipId,
+          treeIndex: `0x${membershipId}`,
           chainId: network.chainId.toString(),
           rateLimit: decodedData.membershipRateLimit.toNumber()
         }
@@ -602,7 +602,7 @@ export class RLNContract {
         identity,
         membership: {
           address,
-          treeIndex: membershipId,
+          treeIndex: `0x${membershipId}`,
           chainId: network.chainId.toString(),
           rateLimit: decodedData.membershipRateLimit.toNumber()
         }
diff --git a/packages/rln/src/contract/rln_light_contract.ts b/packages/rln/src/contract/rln_light_contract.ts
index 3b859f6213..0798633979 100644
--- a/packages/rln/src/contract/rln_light_contract.ts
+++ b/packages/rln/src/contract/rln_light_contract.ts
@@ -395,7 +395,7 @@ export class RLNLightContract {
         identity,
         membership: {
           address,
-          treeIndex: membershipId,
+          treeIndex: `0x${membershipId}`,
           chainId: network.chainId.toString(),
           rateLimit: decodedData.membershipRateLimit.toNumber()
         }
@@ -526,7 +526,7 @@ export class RLNLightContract {
         identity,
         membership: {
           address,
-          treeIndex: membershipId,
+          treeIndex: `0x${membershipId}`,
           chainId: network.chainId.toString(),
           rateLimit: decodedData.membershipRateLimit.toNumber()
         }
diff --git a/packages/rln/src/keystore/types.ts b/packages/rln/src/keystore/types.ts
index 41eccab09a..f9b75f56fe 100644
--- a/packages/rln/src/keystore/types.ts
+++ b/packages/rln/src/keystore/types.ts
@@ -10,7 +10,7 @@ export type Password = string | Uint8Array;
 export type MembershipInfo = {
   chainId: string;
   address: string;
-  treeIndex: string;
+  treeIndex: `0x${string}`; // hexadecimal string
   rateLimit: number;
 };