Errors in plugin

Assemblefile

// Removed everything that doesn't have to do with this specific thing
var assemble = require('assemble'),
    bufferPages = require('./pages.js');

assemble.task('buffer', function () {
    assemble.src('*.md')
        .pipe(bufferPages())
        .pipe(assemble.dest('data'));
});

Error

/home/marie/projects/smaty.se/node_modules/gulp-util/lib/PluginError.js:102
    return '    ' + prop + ': ' + self[prop];
                                ^
TypeError: Cannot convert object to primitive value
    at stringifyProperty (/home/marie/projects/smaty.se/node_modules/gulp-util/lib/PluginError.js:102:33)
    at Array.map (native)
    at PluginError._messageDetails (/home/marie/projects/smaty.se/node_modules/gulp-util/lib/PluginError.js:101:27)
    at PluginError._messageWithDetails (/home/marie/projects/smaty.se/node_modules/gulp-util/lib/PluginError.js:80:22)
    at PluginError.toString (/home/marie/projects/smaty.se/node_modules/gulp-util/lib/PluginError.js:124:16)
    at Stream.<anonymous> (/home/marie/projects/smaty.se/node_modules/assemble/node_modules/assemble-utils/node_modules/session-cache/node_modules/continuation-local-storage/context.js:78:13)
    at Stream.emit (events.js:104:17)
    at Stream.emitted [as emit] (/home/marie/projects/smaty.se/node_modules/assemble/node_modules/assemble-utils/node_modules/session-cache/node_modules/continuation-local-storage/node_modules/emitter-listener/listener.js:122:21)
    at DestroyableTransform.handleEnd (/home/marie/projects/smaty.se/node_modules/assemble/node_modules/event-stream/node_modules/duplexer/index.js:81:21)
    at DestroyableTransform.emit (events.js:129:20)

index.md

shortTitle	pageTitle	layout	hasTwitter	tweetables	excerpt
Home	So, what's this about?	index	true	You like my writing? Sign up for my monthly newsletter! Not everything is as it seems. Magic exists in the shadows … Norns weave destiny; I weave stories. Mystic threads from then to now	Småty is a term for supernatural critters, and if you're interested in magical realism or Scandinavial folklore, this is a good place to start.

This page is about a world that exists in my imagination, at least mainly. It’s called Småty since that’s a dialectal word from Bohuslän, Sweden, for the various supernatural critters in the world. If you are interested in Viking historical fiction and contemporary magical realism inspired by Scandinavian folklore (including the Norse gods), this is a good place to be.

The stories are a mix of historical (mainly the iron age, both before the so-called Viking age and during) and contemporary, something in the magical realism line. They are all set in Sweden, generally with connection to Bohuslän, which in old times was known as either Alfheim or Ranrike. I prefer Ranrike, so that is the term used in my world.

Check out the characters (which includes interviews and bios), the locations and their real-world inspiration, or the stories, which has excerpts of longer works as well as both microfiction (up to 140 characters) and flash fiction (up to about 1000 words).

package.json

{
  "devDependencies": {
    "assemble": "git://github.com/assemble/assemble.git#v0.6.0",
    "through2": "^0.6.3",
    "vinyl": "^0.4.6"
  }
}

pages.js

'use strict';

var through = require('through2'),
    path = require('path'),
    entries = [];
module.exports = function () {
    return through.obj(function (file, enc, cb) {
        var fp = file.path,
            extname = path.extname(fp),
            filename = path.basename(fp, extname),
            link = path.dirname(fp) + '/' + filename + '/',
            tweetables = file.data.tweetables || [];

        tweetables = tweetables.filter(function (tweetable) {
            return tweetable.length >= 50 && tweetable.length <= 70;
        });

        console.log(tweetables);
        cb();
    },
    function (cb) {
        var contents = '{}',
            File = require('vinyl'),
            file = new File({
                cwd: process.cwd(),
                base: '/data/',
                path: '/data/files.json',
                contents: new Buffer(contents)
            });

        this.push(file);
        cb();
    });
};