diff --git a/src/background/actions.ts b/src/background/actions.ts
index dfd6a6b..5ae145b 100644
--- a/src/background/actions.ts
+++ b/src/background/actions.ts
@@ -19,14 +19,14 @@ export function redirectTab(tab: chrome.tabs.Tab, url: string) {
                 let queryErr: any;
                 let newURL = await queryUrl(tab).catch(e => queryErr = e);
                 if (queryErr) {
-                    return Promise.reject(queryErr);
+                    throw queryErr;
                 }
                 if (newURL == url) return url;
                 if (
                     tryCount % 1 == 0 &&
                     !confirm('Cannot navigate to target url. \nPress OK to continue, Cancel to stop.')
                 ) {
-                    return Promise.reject("Tasks stopped by user.");
+                    throw "Tasks stopped by user.";
                 }
                 return undefined;
             }
diff --git a/src/background/messaging.ts b/src/background/messaging.ts
index 3429250..d5c3357 100644
--- a/src/background/messaging.ts
+++ b/src/background/messaging.ts
@@ -50,40 +50,18 @@ export function sendMessage<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 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);
-                                return;
-                            }
-                        } else {
-                            result = pms;
-                        }
-                    }
+                let [result, error] = await checkResponse(dataChecker, r, err, count);
+                if (error) {
+                    reject(error);
+                    return;
                 }
-
-                let flag = result !== undefined && result !== null;
+                let flag = result !== undefined;
                 if (log) logger.info(log, flag ? '(OK)' : '(failed)');
                 if (flag) {
                     resolve(result);
                 } else {
                     setTimeout(() => {
+                        logger.debug('Invalid response', r, 'retry...');
                         loop();
                     }, interval);
                 }
@@ -92,6 +70,39 @@ export function sendMessage<T>(
     });
 }
 
+async function checkResponse<T>(
+    dataChecker: ResponseChecker<T>,
+    response: Response<T>,
+    error: chrome.runtime.LastError,
+    tryCount: number
+): Promise<[T, string]> {
+    // response could be undefined if the content script is interrupted.
+    // don't check, tell sendMessage to retry.
+    if (!response) return [undefined, undefined];
+    if (!dataChecker) {
+        return [response.result, response.error];
+    }
+    let result: T;
+    let pms: T | Promise<T>;
+    try {
+        pms = dataChecker(response, error, tryCount);
+    } catch (err) {
+        return [undefined, err];
+    }
+    // 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) {
+            return [undefined, checkerError];
+        }
+    } else {
+        result = pms;
+    }
+    return [result, undefined];
+}
+
 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;
diff --git a/src/background/task.ts b/src/background/task.ts
index d630e19..93230c5 100644
--- a/src/background/task.ts
+++ b/src/background/task.ts
@@ -103,8 +103,8 @@ export class Task {
         messageSubscribers.addListener(Actions.REPORT_NEW_PAGE, listener);
     }
     async execute(tab: chrome.tabs.Tab, upstreamData?: ExtractResult): Promise<void> {
-        if (!tab) return Promise.reject("No tab to execute the task.");
-        if (this._running) return Promise.reject("The task is running. Please wait...");
+        if (!tab) throw "No tab to execute the task.";
+        if (this._running) throw "The task is running. Please wait...";
         this._running = true;
         let urls = this._urls
         if (!urls.length) {
@@ -140,7 +140,7 @@ export class Task {
         ).catch(
             e => {
                 this._running = false;
-                return Promise.reject(e);
+                throw e;
             }
         );
     }
@@ -152,7 +152,7 @@ export class Task {
         return pm;
     }
     private runningCheck(fn: () => Promise<any>): Promise<any> {
-        if (!this._running) return Promise.reject("The task is stopped by user.");
+        if (!this._running) throw "The task is stopped by user.";
         return fn();
     }
     private saveResult(results, key) {