diff --git a/src/client/client.ts b/src/client/client.ts
index 7091bd4..8e50ea3 100644
--- a/src/client/client.ts
+++ b/src/client/client.ts
@@ -31,6 +31,7 @@ import * as phase0 from "@lodestar/types/phase0";
 import * as bellatrix from "@lodestar/types/bellatrix";
 import { init } from "@chainsafe/bls/switchable";
 import NodeCache from "node-cache";
+import { Mutex } from "async-mutex";
 
 export default class Client {
   latestCommittee?: Uint8Array[];
@@ -46,6 +47,8 @@ export default class Client {
   private beaconChainAPIURL: string;
   private store: IStore;
 
+  private syncMutex = new Mutex();
+
   constructor(prover: IProver, store: IStore, beaconUrl: string) {
     this.prover = prover;
     this.store = store;
@@ -228,11 +231,15 @@ export default class Client {
   }
 
   protected async _sync() {
+    await this.syncMutex.acquire();
+
     const currentPeriod = this.getCurrentPeriod();
     if (currentPeriod > this._latestPeriod) {
       this.latestCommittee = await this.syncFromGenesis();
       this._latestPeriod = currentPeriod;
     }
+
+    this.syncMutex.release();
   }
 
   // committee and prover index of the first honest prover