diff --git a/ext/js/yomichan.js b/ext/js/yomichan.js
index c65e2f7d..33e84df1 100644
--- a/ext/js/yomichan.js
+++ b/ext/js/yomichan.js
@@ -49,12 +49,17 @@ class Yomichan extends EventDispatcher {
     constructor() {
         super();
 
-        this._extensionName = 'Yomichan';
         try {
             const manifest = chrome.runtime.getManifest();
             this._extensionName = `${manifest.name} v${manifest.version}`;
         } catch (e) {
-            // NOP
+            this._extensionName = 'Yomichan';
+        }
+
+        try {
+            this._extensionUrlBase = chrome.runtime.getURL('/');
+        } catch (e) {
+            this._extensionUrlBase = null;
         }
 
         this._isBackground = null;
@@ -148,11 +153,7 @@ class Yomichan extends EventDispatcher {
      * @returns true if the URL is an extension URL, false otherwise.
      */
     isExtensionUrl(url) {
-        try {
-            return url.startsWith(chrome.runtime.getURL('/'));
-        } catch (e) {
-            return false;
-        }
+        return this._extensionUrlBase !== null && url.startsWith(this._extensionUrlBase);
     }
 
     /**