AssertionError [ERR_ASSERTION]: Task function must be specifie

[ghostinbits]

When using gulp, this error occurred:

assert.js:351
    throw err;
    ^

AssertionError [ERR_ASSERTION]: Task function must be specified
    at Gulp.set [as _setTask] (~/photography/node_modules/undertaker/lib/set-task.js:10:3)
    at Gulp.task (~/photography/node_modules/undertaker/lib/task.js:13:8)
    at Object.<anonymous> (~/photography/gulpfile.js:24:6)
    at Module._compile (internal/modules/cjs/loader.js:738:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:749:10)
    at Module.load (internal/modules/cjs/loader.js:630:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:570:12)
    at Function.Module._load (internal/modules/cjs/loader.js:562:3)
    at Module.require (internal/modules/cjs/loader.js:667:17)
    at require (internal/modules/cjs/helpers.js:20:18)

How should I fix this? Thanks!