diff --git a/src/background/controllers/command.js b/src/background/controllers/command.js
index 41057e0..befab42 100644
--- a/src/background/controllers/command.js
+++ b/src/background/controllers/command.js
@@ -48,6 +48,10 @@ export default class CommandController {
     let trimmed = line.trimStart();
     let words = trimmed.split(/ +/);
     let name = words[0];
+    if (words[0].length === 0) {
+      return Promise.resolve();
+    }
+
     let keywords = trimmed.slice(name.length).trimStart();
     switch (words[0]) {
     case 'o':
@@ -85,5 +89,6 @@ export default class CommandController {
     case 'set':
       return this.commandIndicator.set(keywords);
     }
+    throw new Error(words[0] + ' command is not defined');
   }
 }
diff --git a/src/background/infrastructures/content-message-listener.js b/src/background/infrastructures/content-message-listener.js
index 2fa5f54..58716fb 100644
--- a/src/background/infrastructures/content-message-listener.js
+++ b/src/background/infrastructures/content-message-listener.js
@@ -18,12 +18,19 @@ export default class ContentMessageListener {
 
   run() {
     browser.runtime.onMessage.addListener((message, sender) => {
-      return this.onMessage(message, sender).catch((e) => {
+      try {
+        return this.onMessage(message, sender).catch((e) => {
+          return browser.tabs.sendMessage(sender.tab.id, {
+            type: messages.CONSOLE_SHOW_ERROR,
+            text: e.message,
+          });
+        });
+      } catch (e) {
         return browser.tabs.sendMessage(sender.tab.id, {
           type: messages.CONSOLE_SHOW_ERROR,
           text: e.message,
         });
-      });
+      }
     });
   }