Gulp build command gives me this error message - gulp

Question

$ gulp build
[02:36:37] Using gulpfile C:\xampp\htdocs\melodic\gulpfile.js
[02:36:37] Starting 'build'...
[02:36:37] Starting 'clean:dist'...
[02:36:37] Finished 'clean:dist' after 4.63 ms
[02:36:37] Starting 'sass'...
[02:36:37] Starting 'useref'...
[02:36:37] Starting 'img'...
[02:36:37] Starting 'cleancss'...
[02:36:37] Finished 'cleancss' after 37 ms
C:\xampp\htdocs\melodic\node_modules\gulp-useref\node_modules\vinyl-
fs\lib\src\index.js:20
throw new Error('Invalid glob argument: ' + glob);
^
Error: Invalid glob argument:
at Object.src (C:\xampp\htdocs\melodic\node_modules\gulp-
useref\node_modules\vinyl-fs\lib\src\index.js:20:11)
at DestroyableTransform.addAssetsToStream
(C:\xampp\htdocs\melodic\node_modules\gulp-useref\index.js:62:15)
at C:\xampp\htdocs\melodic\node_modules\gulp-useref\index.js:124:31
at Array.forEach (native)
at DestroyableTransform.processAssets
(C:\xampp\htdocs\melodic\node_modules\gulp-useref\index.js:115:11)
at C:\xampp\htdocs\melodic\node_modules\gulp-useref\index.js:178:31
at Stream.<anonymous> (C:\xampp\htdocs\melodic\node_modules\gulp-
useref\node_modules\event-stream\index.js:318:20)
at _end (C:\xampp\htdocs\melodic\node_modules\through\index.js:65:9)
at Stream.stream.end
(C:\xampp\htdocs\melodic\node_modules\through\index.js:74:5)
at DestroyableTransform.onend
(C:\xampp\htdocs\melodic\node_modules\through2\node_modules\readable-
stream\lib\_stream_readable.js:577:10)
I have tried to remove the running tasks one by one but to no avail. What throws me off also is the fact that it seems to run through all the tasks and even finishing them and then crashing as the last task is finished. Thanks in advance.
EDIT: Here is my gulpfile as I forgot to include it.
var gulp = require('gulp');
// Requires the gulp-sass plugin
var sass = require('gulp-sass');
//Requires browser synch
var browserSync = require('browser-sync').create();
//Requires userref
var useref = require('gulp-useref');
//Requires uglify
var uglify = require('gulp-uglify');
var gulpIf = require('gulp-if');
//Requires cssnano
var cssnano = require('gulp-cssnano');
//Requires imagemin
var imagemin = require('gulp-imagemin');
//Requires cache
var cache = require('gulp-cache');
//Requires del
var del = require('del');
//Requires run-sequence
var runSequence = require('run-sequence');
//requires postcss
var postcss = require('gulp-postcss');
//ACTIVE COMMANDS
/*Run 'gulp browserSync' in commandline.
This task automatically reloads the broswer upon each save of document*/
gulp.task('browserSync', function() {
browserSync.init({
injectChanges: true,
server: {
baseDir: "./app"
},
})
});
/*Run 'gulp sass' in command line.
THIS TASK GETS ALL SCSS IMPORTED INTO THE app.scss FILE AND CONVERTS IT INTO app.css.
THIS TASK IS A ONE TIME CONVERSTION*/
gulp.task('sass', function() {
return gulp.src('app/scss/**/*.scss') // Gets all files ending with .scss in app/scss
.pipe(sass())
.pipe(gulp.dest('app/css'))
.pipe(browserSync.stream({match: '**/*.css'}));
});
/*Run 'gulp watch' in command line.
THIS TASK GETS ALL SCSS AND OTHER FILE TYPES AND CONVERTS THEM ACITVELY.
THIS TASK IS ONGOING*/
gulp.task('watch', ['browserSync', 'sass'], function (){
gulp.watch('app/scss/**/*.scss', ['sass']);
});
//PRODUCTION/FINAL COMMANDS
/*Run 'gulp useref' in command line.
THIS TASK GETS ALL CSS AND OTHER FILE TYPES AND JOINS DIFFERENT FILES AND MINIMIZES THEM IN THE DIST FOLDER.
*/
gulp.task('useref', function(){
return gulp.src('app/*.html')
.pipe(useref())
// Minifies only if it's a JavaScript file
.pipe(gulpIf('*.js', uglify()))
// Minifies only if it's a CSS file
.pipe(gulpIf('*.css', cssnano()))
.pipe(gulp.dest('dist'))
});
gulp.task('cleancss', function () {
return gulp.src('./src/*.css')
.pipe(postcss())
.pipe(gulp.dest('./dest'));
});
/*Run 'gulp img' in command line.
MINIMIZSES PHOTOS TO THE DIST FOLDER*/
gulp.task('img', function(){
return gulp.src('app/img/**/*.+(png|jpg|gif|svg)')
.pipe(imagemin({
// Setting interlaced to true
interlaced: true
}))
// Caching images that ran through imagemin
.pipe(cache(imagemin({
interlaced: true
})))
.pipe(gulp.dest('dist/img'))
});
/*Run 'gulp fonts' in command line.
CLEANS THE DIST DIRECTORY FOR UNUSED FILES*/
gulp.task('clean:dist', function() {
return del.sync('dist');
});
/*Run 'gulp fonts' in command line.
CLEANS THE IMAGE CACHE CREATED EARLIER*/
gulp.task('cache:clear', function (callback) {
return cache.clearAll(callback)
});
gulp.task('build', function (callback) {
runSequence('clean:dist',
['sass', 'useref', 'img', 'cleancss'],
callback
)
});
gulp.task('default', function (callback) {
runSequence(['sass','browserSync', 'watch'],
callback
)
});
EDIT: Included my gulpfile for further details.