String.prototype[Symbol.iterator] does RequireObjectCoercible(this)

src/js/string-iterator.js

Index: src/js/string-iterator.js
diff --git a/src/js/string-iterator.js b/src/js/string-iterator.js
index 3c331dd1a2da4e99dc0e84be7c9ffa091f01acad..af9af31efd884ef1322c625ed4fc9676e7dd1383 100644
--- a/src/js/string-iterator.js
+++ b/src/js/string-iterator.js
@@ -32,6 +32,7 @@ function StringIterator() {}
 
 // 21.1.5.1 CreateStringIterator Abstract Operation
 function CreateStringIterator(string) {
+  CHECK_OBJECT_COERCIBLE(string, 'String.prototype[Symbol.iterator]');
   var s = TO_STRING(string);
   var iterator = new StringIterator;
   SET_PRIVATE(iterator, stringIteratorIteratedStringSymbol, s);