onLast prevents task completing

[benfrain]

I'm unsure if this is my own lack of understanding. I'm using onLast to send a notify when a task completes on a bunch of files. Here is the relevant task:

var sourceCSS = "preCSS/**/*.css";
var outputCSS = "css/"

gulp.task("styles", function () {

    //PostCSS related
    var postcss = require("gulp-postcss");
    var postcssImport = require("postcss-import");
    var autoprefixer = require("autoprefixer");
    var simpleVars = require("postcss-simple-vars");
    var mixins = require("postcss-mixins");
    var nested = require("postcss-nested");
    //Utils
    var gutil = require("gulp-util");
    // var rename = require("gulp-rename");
    var notify = require("gulp-notify");

    var processors = [
    postcssImport({ glob: true }),
    mixins,
    simpleVars,
    nested];

    return gulp.src(sourceCSS)

    // We always want PostCSS to run
    .pipe(postcss(processors).on("error", gutil.log))

    // Set the destination for the CSS file
    .pipe(gulp.dest(outputCSS)

    // Notify me
    //.pipe(notify({
    //    "onLast": true,
    //    "title": "Styles built from preCSS",
    //    "message": "Boom-chikka-wha-wha"
    //}))
    );

});

However, with the piped notify un-commented I get the notify but not all the files in the job are processed (some files in the preCSS/ folder don't make it to the css folder).

Unsure if this is an issue with my usage of onLast or an actual bug. Using Windows 7, Gulp v3.9.0, node v0.12.7, gulp-notify v 2.2.0