diff --git a/ext/fg/js/popup.js b/ext/fg/js/popup.js
index 42f08afa..99610e17 100644
--- a/ext/fg/js/popup.js
+++ b/ext/fg/js/popup.js
@@ -539,19 +539,10 @@ class Popup {
         };
     }
 
-    static _isOnExtensionPage() {
-        try {
-            const url = chrome.runtime.getURL('/');
-            return window.location.href.substring(0, url.length) === url;
-        } catch (e) {
-            // NOP
-        }
-    }
-
     static async _injectStylesheet(id, type, value, useWebExtensionApi) {
         const injectedStylesheets = Popup._injectedStylesheets;
 
-        if (Popup._isOnExtensionPage()) {
+        if (yomichan.isExtensionUrl(window.location.href)) {
             // Permissions error will occur if trying to use the WebExtension API to inject
             // into an extension page.
             useWebExtensionApi = false;
diff --git a/ext/mixed/js/core.js b/ext/mixed/js/core.js
index 2d11c11a..6a3298fc 100644
--- a/ext/mixed/js/core.js
+++ b/ext/mixed/js/core.js
@@ -316,6 +316,15 @@ const yomichan = (() => {
             this.trigger('orphaned', {error});
         }
 
+        isExtensionUrl(url) {
+            try {
+                const urlBase = chrome.runtime.getURL('/');
+                return url.substring(0, urlBase.length) === urlBase;
+            } catch (e) {
+                return false;
+            }
+        }
+
         getTemporaryListenerResult(eventHandler, userCallback, timeout=null) {
             if (!(
                 typeof eventHandler.addListener === 'function' &&