diff --git a/src/background/actions.ts b/src/background/actions.ts
index c539ee6..dfd6a6b 100644
--- a/src/background/actions.ts
+++ b/src/background/actions.ts
@@ -1,5 +1,5 @@
 import { Actions, Request } from "../common";
-import { sendMessage } from "./messaging";
+import { sendMessage, ResponseChecker } from "./messaging";
 import { logger } from "./logger";
 
 /**
@@ -15,7 +15,7 @@ export function redirectTab(tab: chrome.tabs.Tab, url: string) {
                 action: Actions.GOTO_URL,
                 url: url
             }
-            let checker = async (u, err, tryCount): Promise<string> => {
+            let checker: ResponseChecker<string> = async (r, err, tryCount): Promise<string> => {
                 let queryErr: any;
                 let newURL = await queryUrl(tab).catch(e => queryErr = e);
                 if (queryErr) {
@@ -42,13 +42,16 @@ export function redirectTab(tab: chrome.tabs.Tab, url: string) {
  * @param {Array<string>} fieldSelectors fields selectors for selecting fields (data columns) under each item
  * @returns {Promise<string[]>} a promise of extracted data
  */
-export function extractTabData(tab: chrome.tabs.Tab, itemsSelector: string, fieldSelectors: string[], askOnfail?: boolean) {
-    let req = {
+export function extractTabData(tab: chrome.tabs.Tab, itemsSelector: string, fieldSelectors: string[], expectedURL?: string, askOnfail?: boolean) {
+    let req: Request = {
         action: Actions.EXTRACT,
         itemsSelector: itemsSelector,
-        fieldSelectors: fieldSelectors
+        fieldSelectors: fieldSelectors,
+        url: expectedURL,
     }
-    let checker = (result, err, tryCount) => {
+    let checker: ResponseChecker<string[][]> = (response, err, tryCount) => {
+        if (response.error) throw response.error;
+        let result = response.result;
         if (!result || !result.length) {
             if (
                 tryCount % 20 == 0 && (
@@ -76,7 +79,9 @@ export async function ping(tab, count = 1) {
     let req = {
         action: Actions.PING
     }
-    let checker = (r: string, e, c) => r == "pong" ? r : undefined;
+    let checker: ResponseChecker<string> = (r, e, c) =>
+        r.result == "pong" ? r.result : undefined;
+
     let pong = await sendMessage<string>(tab, req, 'Check tab availability...', checker, 1000, count).catch(() => { });
     return pong == "pong";
 }
diff --git a/src/background/messaging.ts b/src/background/messaging.ts
index d01de8d..3429250 100644
--- a/src/background/messaging.ts
+++ b/src/background/messaging.ts
@@ -1,7 +1,11 @@
-import { Request, Actions } from "../common";
+import { Request, Actions, Response } from "../common";
 import { getTabByID } from "./actions";
 import { logger } from "./logger";
 
+
+export type ResponseCheckerSync<T> = (r: Response<T>, err: chrome.runtime.LastError, count: number) => T;
+export type ResponseCheckerAsync<T> = (r: Response<T>, err: chrome.runtime.LastError, count: number) => Promise<T>;
+export type ResponseChecker<T> = ResponseCheckerSync<T> | ResponseCheckerAsync<T>;
 /**
  *  Sending a message to target tab repeatedly until the response is not undefined.
  * @param {object} tab the table where to send the message
@@ -19,7 +23,7 @@ export function sendMessage<T>(
     tab: chrome.tabs.Tab,
     req,
     log?: string,
-    dataChecker?: (r: T, err: chrome.runtime.LastError, count: number) => T | Promise<T>,
+    dataChecker?: ResponseChecker<T>,
     interval?: number,
     limit?: number
 ) {
@@ -43,30 +47,34 @@ export function sendMessage<T>(
                 return;
             }
             count++;
-            chrome.tabs.sendMessage(tab.id, req, async (r: T) => {
+            chrome.tabs.sendMessage(tab.id, req, async (r: Response<T>) => {
                 // check error but do nothing until dataChecker.
                 let err = chrome.runtime.lastError;
-                let result: T = r;
+                let result: T;
+                // r could be undefined if the content script is interrupted.
+                if (r) {
+                    result = r.result;
 
-                if (dataChecker) {
-                    let pms: T | Promise<T>;
-                    try {
-                        pms = dataChecker(r, err, count);
-                    } catch (error) {
-                        reject(error);
-                        return;
-                    }
-                    // don't catch if it's not a Promise
-                    if (pms instanceof Promise) {
-                        let checkerError: any;
-                        pms = pms.catch(e => checkerError = e);
-                        result = await pms;
-                        if (checkerError) {
-                            reject(checkerError);
+                    if (dataChecker) {
+                        let pms: T | Promise<T>;
+                        try {
+                            pms = dataChecker(r, err, count);
+                        } catch (error) {
+                            reject(error);
                             return;
                         }
-                    } else {
-                        result = pms;
+                        // don't catch if it's not a Promise
+                        if (pms instanceof Promise) {
+                            let checkerError: any;
+                            pms = pms.catch(e => checkerError = e);
+                            result = await pms;
+                            if (checkerError) {
+                                reject(checkerError);
+                                return;
+                            }
+                        } else {
+                            result = pms;
+                        }
                     }
                 }
 
@@ -84,7 +92,10 @@ export function sendMessage<T>(
     });
 }
 
-export type ActionSubscriber = (request: Request, sender: chrome.runtime.MessageSender, sendResponse: (response?: any) => void) => void | Promise<void>;
+export type ActionSubscriberSync = (request: Request, sender: chrome.runtime.MessageSender, sendResponse: (response?: any) => void) => void;
+export type ActionSubscriberAsync = (request: Request, sender: chrome.runtime.MessageSender, sendResponse: (response?: any) => void) => Promise<void>;
+export type ActionSubscriber = ActionSubscriberSync | ActionSubscriberAsync;
+
 class MessageSubscribers {
     private listeners: { [key: number]: ActionSubscriber[] } = {};
     addListener(action: Actions, subscriber: ActionSubscriber) {
diff --git a/src/background/task.ts b/src/background/task.ts
index 9954b1e..d630e19 100644
--- a/src/background/task.ts
+++ b/src/background/task.ts
@@ -73,6 +73,7 @@ export class Task {
             logger.info("No window to watch...");
             return;
         }
+        let watchTaskID = 0;
         let listener: ActionSubscriber = async (request, sender, sendResponse) => {
             let findWindow = await getWindowByID(window.id);
             if (!findWindow) {
@@ -82,17 +83,20 @@ export class Task {
             }
             // only watch current window.
             if (sender.tab.windowId != window.id) return;
+            let taskID = watchTaskID++;
+            logger.info(`Watcher #${taskID} starts.`);
             let pm = this.makeOptionalTasks(sender.tab);
             return pm.then(
-                () => extractTabData(sender.tab, this._itemsSelector, this._fieldSelectors, false)
+                () => extractTabData(sender.tab, this._itemsSelector, this._fieldSelectors, sender.tab.url, false)
             ).then(
                 results => {
                     if (results && results.length) {
                         this.saveResult(results, sender.tab.url);
                     }
+                    logger.info(`Watcher #${taskID} ends.`);
                 }
             ).catch(
-                e => logger.error(e)
+                e => logger.error(`Watcher #${taskID} ends with:`, e)
             )
         }
         this._listeners.push(listener);
diff --git a/src/common.ts b/src/common.ts
index 58f5e7a..e07ca76 100644
--- a/src/common.ts
+++ b/src/common.ts
@@ -20,4 +20,9 @@ export interface Request {
     url?: string
     fileName?: string
     state?: string
+}
+
+export interface Response<T> {
+    result: T;
+    error: string;
 }
\ No newline at end of file
diff --git a/src/content/actions.ts b/src/content/actions.ts
index 7041206..1ff86db 100644
--- a/src/content/actions.ts
+++ b/src/content/actions.ts
@@ -1,4 +1,7 @@
-export function extract(itemsSelector: string, fieldSelectors: string[]): string[][] {
+export function extract(itemsSelector: string, fieldSelectors: string[], expectedURL: string): string[][] {
+    if (expectedURL && location.href != expectedURL) {
+        throw 'Target tab URL changed, aborting...';
+    }
     // since some elements may be loaded asynchronously.
     // if one field is never found, we should return undefined,
     // so that senders can detect to retry until elements loaded.
diff --git a/src/content/index.ts b/src/content/index.ts
index cb0d491..f22cbcc 100644
--- a/src/content/index.ts
+++ b/src/content/index.ts
@@ -1,4 +1,4 @@
-import { Request, Actions } from '../common';
+import { Request, Actions, Response } from '../common';
 import { scrollToBottom, extract } from './actions';
 
 let asleep = false;
@@ -20,30 +20,56 @@ chrome.runtime.sendMessage(<Request>{
     action: Actions.REPORT_NEW_PAGE,
 });
 
-async function doAction(request: Request, sender: chrome.runtime.MessageSender) {
-    switch (request.action) {
-        case Actions.EXTRACT:
-            let data = extract(request.itemsSelector, request.fieldSelectors);
-            return data;
-        case Actions.GOTO_URL:
-            window.location.replace(request.url);
-            // should not recieve any request until the page & script reload
-            asleep = true;
-            return request.url;
-        case Actions.PING:
-            return "pong";
-        case Actions.QUERY_URL:
-            return window.location.href;
-        case Actions.SCROLL_BOTTOM:
-            return scrollToBottom();
-        case Actions.SLEEP:
-            asleep = true;
-            return "Content script is sleeping.";
-        case Actions.WAKEUP:
-            asleep = false;
-            return "Content script is available.";
-        default:
-            break;
+async function doAction(request: Request, sender: chrome.runtime.MessageSender): Promise<Response<any>> {
+    let result: any;
+    let error: string;
+    try {
+        switch (request.action) {
+            case Actions.EXTRACT:
+                result = extract(request.itemsSelector, request.fieldSelectors, request.url);
+                break;
+            case Actions.GOTO_URL:
+                window.location.replace(request.url);
+                // should not recieve any request until the page & script reload
+                asleep = true;
+                result = request.url;
+                break;
+            case Actions.PING:
+                result = "pong";
+                break;
+            case Actions.QUERY_URL:
+                result = window.location.href;
+                break;
+            case Actions.SCROLL_BOTTOM:
+                result = scrollToBottom();
+                break;
+            case Actions.SLEEP:
+                asleep = true;
+                result = "Content script is sleeping.";
+                break;
+            case Actions.WAKEUP:
+                asleep = false;
+                result = "Content script is available.";
+                break;
+            default:
+                error = 'Unsupported action.'
+                break;
+        }
+    } catch (err) {
+        if (err instanceof Error) {
+            error = err.message;
+        } else {
+            error = err;
+        }
     }
+    return newResponse(result, error);
 }
 
+
+function newResponse<T>(result: T, err?: string): Response<T> {
+    let r: Response<T> = {
+        result: result,
+        error: err,
+    }
+    return r;
+}
\ No newline at end of file