diff --git a/ext/mixed/js/display.js b/ext/mixed/js/display.js
index 575011fd..228e554a 100644
--- a/ext/mixed/js/display.js
+++ b/ext/mixed/js/display.js
@@ -539,7 +539,7 @@ class Display {
 
     static getKeyFromEvent(event) {
         const key = event.key;
-        return key.length === 1 ? key.toUpperCase() : key;
+        return (typeof key === 'string' ? (key.length === 1 ? key.toUpperCase() : key) : '');
     }
 }