《开发指南》express4.0 实现注册，出错...

控制台报错信息： /Users/victor/mymicroblog/node_modules/mongodb/lib/mongodb/db.js:296 throw err; ^ Error: Cannot use a writeConcern without a provided callback at Db.ensureIndex (/Users/victor/mymicroblog/node_modules/mongodb/lib/mongodb/db.js:1426:11) at Collection.ensureIndex (/Users/victor/mymicroblog/node_modules/mongodb/lib/mongodb/collection/index.js:65:11) at /Users/victor/mymicroblog/models/user.js:24:16 at Db.collection (/Users/victor/mymicroblog/node_modules/mongodb/lib/mongodb/db.js:502:44) at /Users/victor/mymicroblog/models/user.js:19:6 at /Users/victor/mymicroblog/node_modules/mongodb/lib/mongodb/db.js:293:11 at process._tickCallback (node.js:419:13) Program node app.js exited with code 8