diff --git a/guacamole-common-js/src/main/webapp/modules/Keyboard.js b/guacamole-common-js/src/main/webapp/modules/Keyboard.js
index 163aef0cc..37145307e 100644
--- a/guacamole-common-js/src/main/webapp/modules/Keyboard.js
+++ b/guacamole-common-js/src/main/webapp/modules/Keyboard.js
@@ -640,10 +640,15 @@ Guacamole.Keyboard = function(element) {
      * @param {String} keyIdentifier The legacy keyIdentifier from a
      *                               browser keydown/keyup event.
      * @returns {Boolean} true if the keyIdentifier looks sane, false if
-     *                    the keyIdentifier appears incorrectly derived.
+     *                    the keyIdentifier appears incorrectly derived or
+     *                    is missing entirely.
      */
     function key_identifier_sane(keyCode, keyIdentifier) {
 
+        // Missing identifier is not sane
+        if (!keyIdentifier)
+            return false;
+
         // Assume non-Unicode keyIdentifier values are sane
         var unicodePrefixLocation = keyIdentifier.indexOf("U+");
         if (unicodePrefixLocation === -1)