2 files changed, 5 insertions, 0 deletions
diff --git a/src/mempool/order_state_watcher.ts b/src/mempool/order_state_watcher.ts
index 63e812054..d0bf5d89c 100644
--- a/src/mempool/order_state_watcher.ts
+++ b/src/mempool/order_state_watcher.ts
@@ -17,6 +17,7 @@ import {
     OnOrderStateChangeCallback,
     ExchangeEvents,
     TokenEvents,
+    ZeroExError,
 } from '../types';
 import {Web3Wrapper} from '../web3_wrapper';
 
@@ -85,6 +86,9 @@ export class OrderStateWatcher {
      */
     public subscribe(callback: OnOrderStateChangeCallback, numConfirmations: number): void {
         assert.isFunction('callback', callback);
+        if (!_.isUndefined(this._callbackAsync)) {
+            throw new Error(ZeroExError.SubscriptionAlreadyPresent);
+        }
         this._callbackAsync = callback;
         this._eventWatcher.subscribe(this._onMempoolEventCallbackAsync.bind(this), numConfirmations);
     }
diff --git a/src/types.ts b/src/types.ts
index 969f2e96d..246fd8641 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -16,6 +16,7 @@ export enum ZeroExError {
     OutOfGas = 'OUT_OF_GAS',
     NoNetworkId = 'NO_NETWORK_ID',
     SubscriptionNotFound = 'SUBSCRIPTION_NOT_FOUND',
+    SubscriptionAlreadyPresent = 'SUBSCRIPTION_ALREADY_PRESENT',
 }
 
 export enum InternalZeroExError {