elixir.queueTask is undefined

I am fairly new to Laravel 5.2 and Elixir/gulp but I have an issue with queueTask being undefined when I run gulp from the command line.

What I want to do is to extend elixir to delete some files (according to all the documentation I can find, that's what I need to do), so I have this:

var gulp = require('gulp');
var elixir = require("laravel-elixir");
var del = require('del');

elixir.extend("remove", function(path) {
        gulp.task("removeFiles", function() {
            return del(path);
        });
        return this.queueTask("removeFiles");
    });

and then in my mix I have:

    .remove([
        "path/to/file1/filename1",
        "path/to/file2/filename2"
    ])

When I run gulp in the command line, I get:

return this.queueTask("removeFiles");
            ^
TypeError: undefined is not a function

can anyone throw some light on what I am doing wrong please?