diff --git a/lib/browser/collections.js b/lib/browser/collections.js
index c3d89c0b..0b331539 100644
--- a/lib/browser/collections.js
+++ b/lib/browser/collections.js
@@ -117,6 +117,12 @@ export function createCollection(prototype, realmId, info, _mutable) {
         'length': {
             get: getterForProperty('length'),
         },
+        'type': {
+            get: getterForProperty('type'),
+        },
+        'optional': {
+            get: getterForProperty('optional'),
+        },
         '-1': {
             value: undefined,
         },