diff --git a/ext/mixed/js/core.js b/ext/mixed/js/core.js
index 5f9e172d..6d0a5f0e 100644
--- a/ext/mixed/js/core.js
+++ b/ext/mixed/js/core.js
@@ -318,10 +318,10 @@ const yomichan = (() => {
         }
 
         getTemporaryListenerResult(eventHandler, userCallback, timeout=null) {
-            if (
-                typeof eventHandler.addListener === 'undefined' ||
-                typeof eventHandler.removeListener === 'undefined'
-            ) {
+            if (!(
+                typeof eventHandler.addListener === 'function' ||
+                typeof eventHandler.removeListener === 'function'
+            )) {
                 throw new Error('Event handler type not supported');
             }