Cant gulp serve the project

if im using gulp serve i get the following error

`E:\Project\blur-admin>gulp serve
AssertionError [ERR_ASSERTION]: Task function must be specified
at Gulp.set [as _setTask] (E:\Project\blur-admin\node_modules\undertaker\lib\set-task.js:10:3)
at Gulp.task (E:\Project\blur-admin\node_modules\undertaker\lib\task.js:13:8)
at Object. (E:\Project\blur-admin\gulp\build.js:28:6)
at Module._compile (internal/modules/cjs/loader.js:1063:30)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:1092:10)
at Module.load (internal/modules/cjs/loader.js:928:32)
at Function.Module._load (internal/modules/cjs/loader.js:769:14)
at Module.require (internal/modules/cjs/loader.js:952:19)
at require (internal/modules/cjs/helpers.js:88:18)
at E:\Project\blur-admin\gulpfile.js:13:3 {
generatedMessage: false,
code: 'ERR_ASSERTION',
actual: false,
expected: true,
operator: '=='
}`

im using gulp 4.0.0

line 13:3 im gulpfile is:

` require('./gulp/' + file);`

` wrench.readdirSyncRecursive('./gulp').filter(function(file) {
return (/\.(js|coffee)$/i).test(file);
}).map(function(file) {
require('./gulp/' + file);
});`