indexdb

Question

function select(key)  
    {  
           var transaction=db.transaction('Users','readonly');
           transaction.oncomplete = function(e){
            console.log('transaction complete');
          };
          transaction.onerror = function(e){
            console.dir(e);
          };
            var store=transaction.objectStore('Users');
            var index = store.index("username");
            var request=index.openCursor(IDBKeyRange.only(key))
            request.onsuccess=function(e){
                var cursor=e.target.result;
                if(cursor){
                    debugger
                    var student=cursor.value;
                    console.log(student.id);
                    dbData.push(cursor.value);
                    cursor.continue();
                } else {
                     callback && callback();
                }
            }
    }; 

报错信息：index.js:89 Uncaught DOMException: Failed to execute 'index' on 'IDBObjectStore': The specified index was not found.

目前根据key可以进行查询，我想通过username进行查询，但是却报错了，大神帮忙看看是怎么回事