gulp缓存断言错误：缺少路径

这是我的gulpfile.js

var $ = require('gulp-load-plugins')();    

    // Optimize images
        gulp.task('images', function () {
          return gulp.src('app/images/**/*')
            .pipe($.cache($.imagemin({
              progressive: true,
              interlaced: true
            })))
            .pipe(gulp.dest('dist/images'))
            .pipe($.size({title: 'images'}));
        });