diff --git a/CHANGELOG.md b/CHANGELOG.md
index 29475a738..fb62279ce 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,7 @@
 # Changelog
 
 ### vNEXT
+- Operation key is now `string` instead of `number` [PR #176](https://github.com/apollographql/subscriptions-transport-ws/pull/176)
 - ...
 
 ### 0.7.1
diff --git a/src/client.ts b/src/client.ts
index 5a7f81674..697956621 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -228,7 +228,7 @@ export class SubscriptionClient {
     return this.on('reconnecting', callback, context);
   }
 
-  public unsubscribe(opId: number) {
+  public unsubscribe(opId: string) {
     if (this.operations[opId]) {
       delete this.operations[opId];
       this.sendMessage(opId, MessageTypes.GQL_STOP, undefined);
@@ -237,7 +237,7 @@ export class SubscriptionClient {
 
   public unsubscribeAll() {
     Object.keys(this.operations).forEach( subId => {
-      this.unsubscribe(parseInt(subId, 10));
+      this.unsubscribe(subId);
     });
   }
 
@@ -304,7 +304,7 @@ export class SubscriptionClient {
     }
   }
 
-  private executeOperation(options: OperationOptions, handler: (error: Error[], result?: any) => void): number {
+  private executeOperation(options: OperationOptions, handler: (error: Error[], result?: any) => void): string {
     const opId = this.generateOperationId();
     this.operations[opId] = { options: options, handler };
 
@@ -324,7 +324,7 @@ export class SubscriptionClient {
     return opId;
   }
 
-  private buildMessage(id: number, type: string, payload: any) {
+  private buildMessage(id: string, type: string, payload: any) {
     const payloadToReturn = payload && payload.query ?
       {
         ...payload,
@@ -362,7 +362,7 @@ export class SubscriptionClient {
     }];
   }
 
-  private sendMessage(id: number, type: string, payload: any) {
+  private sendMessage(id: string, type: string, payload: any) {
     this.sendMessageRaw(this.buildMessage(id, type, payload));
   }
 
@@ -392,8 +392,8 @@ export class SubscriptionClient {
     }
   }
 
-  private generateOperationId() {
-    return ++this.nextOperationId;
+  private generateOperationId(): string {
+    return String(++this.nextOperationId);
   }
 
   private tryReconnect() {
@@ -404,7 +404,7 @@ export class SubscriptionClient {
     if (!this.reconnecting) {
       Object.keys(this.operations).forEach((key) => {
         this.unsentMessagesQueue.push(
-          this.buildMessage(parseInt(key, 10), MessageTypes.GQL_START, this.operations[key].options),
+          this.buildMessage(key, MessageTypes.GQL_START, this.operations[key].options),
         );
       });
       this.reconnecting = true;
@@ -462,7 +462,7 @@ export class SubscriptionClient {
 
   private processReceivedData(receivedData: any) {
     let parsedMessage: any;
-    let opId: number;
+    let opId: string;
 
     try {
       parsedMessage = JSON.parse(receivedData);
@@ -478,6 +478,7 @@ export class SubscriptionClient {
       ].indexOf(parsedMessage.type) !== -1 && !this.operations[opId]
     ) {
       this.unsubscribe(opId);
+
       return;
     }
 
diff --git a/src/helpers.ts b/src/helpers.ts
index 5b492ffdc..993aa4580 100644
--- a/src/helpers.ts
+++ b/src/helpers.ts
@@ -15,10 +15,10 @@ export function addGraphQLSubscriptions(networkInterface: any, wsClient: Subscri
   }
 
   return assign(networkInterface, {
-    subscribe(request: any, handler: any): number {
+    subscribe(request: any, handler: any): string {
       return wsClient.subscribe(request, handler);
     },
-    unsubscribe(id: number): void {
+    unsubscribe(id: string): void {
       wsClient.unsubscribe(id);
     },
   });
diff --git a/src/test/tests.ts b/src/test/tests.ts
index 3cafe6b22..4f273e864 100644
--- a/src/test/tests.ts
+++ b/src/test/tests.ts
@@ -1727,7 +1727,7 @@ describe('Server', function () {
 
   it('does not send more subscription data after client unsubscribes', function (done) {
     const client4 = new SubscriptionClient(`ws://localhost:${TEST_PORT}/`);
-    let subId: number;
+    let subId: string;
     setTimeout(() => {
       client4.unsubscribe(subId);
     }, 50);