一连串的提示这个错误是什么原因？？？

TypeError: first argument must be a string or Buffer at ServerResponse.OutgoingMessage.end (http.js:978:13) at /var/www/wechat/node_modules/connect/node_modules/express-session/index.js:307:13 at Object._onImmediate (/var/www/wechat/node_modules/connect/node_modules/express-session/session/memory.js:73:11) at processImmediate [as _immediateCallback] (timers.js:330:15) TypeError: first argument must be a string or Buffer at ServerResponse.OutgoingMessage.end (http.js:978:13) at /var/www/wechat/node_modules/connect/node_modules/express-session/index.js:307:13 at Object._onImmediate (/var/www/wechat/node_modules/connect/node_modules/express-session/session/memory.js:73:11) at processImmediate [as _immediateCallback] (timers.js:330:15) TypeError: first argument must be a string or Buffer at ServerResponse.OutgoingMessage.end (http.js:978:13) at /var/www/wechat/node_modules/connect/node_modules/express-session/index.js:307:13 at Object._onImmediate (/var/www/wechat/node_modules/connect/node_modules/express-session/session/memory.js:73:11) at processImmediate [as _immediateCallback] (timers.js:330:15)