diff --git a/src/webhook/handleInput.ts b/src/webhook/handleInput.ts
index 0a5093d3..e41c37be 100644
--- a/src/webhook/handleInput.ts
+++ b/src/webhook/handleInput.ts
@@ -4,72 +4,70 @@ import { extractArticleId } from 'src/lib/sharedUtils';
 import tutorial, { TUTORIAL_STEPS } from './handlers/tutorial';
 import handlePostback from './handlePostback';
 import {
-  ChatbotEvent,
   ChatbotState,
   ChatbotStateHandlerParams,
   ChatbotStateHandlerReturnType,
 } from 'src/types/chatbotState';
 import { Result } from 'src/types/result';
-import { Message } from '@line/bot-sdk';
+import { Message, MessageEvent, TextEventMessage } from '@line/bot-sdk';
 
 /**
  * Given input event and context, outputs the new context and the reply to emit.
  *
- * @param {Object<data>} context The current context of the bot
- * @param {*} event The input event
- * @param {*} userId LINE user ID that does the input
+ * @param context The current context of the bot
+ * @param event The input event
+ * @param userId LINE user ID that does the input
  */
 export default async function handleInput(
   { data = {} },
-  event: ChatbotEvent,
+  event: MessageEvent & { message: TextEventMessage },
   userId: string
 ): Promise<Result> {
   let state: ChatbotState;
   let replies: Message[] = [];
 
-  if (event.input === undefined) {
-    throw new Error('input undefined');
-  }
-
-  if (event.type === 'message') {
-    // Trim input because these may come from other chatbot
-    //
-    const trimmedInput = event.input.trim();
-    const articleId = extractArticleId(trimmedInput);
-    if (articleId) {
-      // Start new session, reroute to CHOOSING_ARTILCE and simulate "choose article" postback event
-      data = {
-        // Start a new session
-        sessionId: Date.now(),
+  // Trim input because these may come from other chatbot
+  //
+  const trimmedInput = event.message.text.trim();
+  const articleId = extractArticleId(trimmedInput);
+  if (articleId) {
+    // Start new session, reroute to CHOOSING_ARTILCE and simulate "choose article" postback event
+    const sessionId = Date.now();
+    return await handlePostback(
+      // Start a new session
+      {
+        sessionId,
         searchedText: '',
-      };
-      event = {
-        type: 'postback',
+      },
+      {
+        state: 'CHOOSING_ARTICLE',
+        sessionId,
         input: articleId,
-      };
-      return await handlePostback({ data }, 'CHOOSING_ARTICLE', event, userId);
-    } else if (event.input === TUTORIAL_STEPS['RICH_MENU']) {
-      state = 'TUTORIAL';
-    } else {
-      // The user forwarded us an new message.
-      // Create a new "search session".
-      //
-      data = {
-        // Used to determine button postbacks and GraphQL requests are from
-        // previous sessions
-        //
-        sessionId: Date.now(),
-      };
-      state = '__INIT__';
-    }
+      },
+      userId
+    );
+  } else if (event.message.text === TUTORIAL_STEPS['RICH_MENU']) {
+    state = 'TUTORIAL';
   } else {
-    state = 'Error';
+    // The user forwarded us an new message.
+    // Create a new "search session".
+    //
+    data = {
+      // Used to determine button postbacks and GraphQL requests are from
+      // previous sessions
+      //
+      sessionId: Date.now(),
+    };
+    state = '__INIT__';
   }
 
-  let params: ChatbotStateHandlerParams | ChatbotStateHandlerReturnType = {
+  const params: ChatbotStateHandlerParams = {
     data,
     state,
-    event,
+    event: {
+      ...event,
+      message: event.message,
+    },
     userId,
     replies,
   };
diff --git a/src/webhook/index.ts b/src/webhook/index.ts
index a6dc9153..700d42c5 100644
--- a/src/webhook/index.ts
+++ b/src/webhook/index.ts
@@ -18,13 +18,9 @@ import {
 import processMedia from './handlers/processMedia';
 import UserSettings from '../database/models/userSettings';
 import { Request } from 'koa';
-import { WebhookEvent } from '@line/bot-sdk';
+import { MessageEvent, TextEventMessage, WebhookEvent } from '@line/bot-sdk';
 import { Result } from 'src/types/result';
-import {
-  ChatbotEvent,
-  Context,
-  PostbackActionData,
-} from 'src/types/chatbotState';
+import { Context, PostbackActionData } from 'src/types/chatbotState';
 
 const userIdBlacklist = (process.env.USERID_BLACKLIST || '').split(',');
 
@@ -133,7 +129,13 @@ const singleUserHandler = async (
 
     result = await processText(
       context,
-      { ...webhookEvent, input },
+      // Make TS happy:
+      // Directly providing `webhookEvent` here can lead to type error
+      // because it cannot correctly narrow down webhookEvent.message to be TextEventMessage.
+      {
+        ...webhookEvent,
+        message: webhookEvent.message,
+      },
       userId,
       req
     );
@@ -224,7 +226,7 @@ const singleUserHandler = async (
 
 async function processText(
   context: { data: Partial<Context> },
-  event: ChatbotEvent,
+  event: MessageEvent & { message: TextEventMessage },
   userId: string,
   req: Request
 ): Promise<Result> {