3 files changed, 8 insertions, 2 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 {
diff --git a/test/event_watcher_test.ts b/test/event_watcher_test.ts
index 208871ea8..a246805a0 100644
--- a/test/event_watcher_test.ts
+++ b/test/event_watcher_test.ts
@@ -23,6 +23,7 @@ describe('EventWatcher', () => {
     let stubs: Sinon.SinonStub[] = [];
     let eventWatcher: EventWatcher;
     let web3Wrapper: Web3Wrapper;
+    const numConfirmations = 0;
     const logA = {
         address: '0x71d271f8b14adef568f8f28f1587ce7271ac4ca5',
         blockHash: null,
@@ -87,7 +88,7 @@ describe('EventWatcher', () => {
                 done();
             }
         };
-        eventWatcher.subscribe(callback);
+        eventWatcher.subscribe(callback, numConfirmations);
     });
     it('correctly computes the difference and emits only changes', (done: DoneCallback) => {
         const initialLogs: Web3.LogEntry[] = [logA, logB];
@@ -121,6 +122,6 @@ describe('EventWatcher', () => {
                 done();
             }
         };
-        eventWatcher.subscribe(callback);
+        eventWatcher.subscribe(callback, numConfirmations);
     });
 });