Mocha 'should' - identical strings that 'should.equal()' but cause Uncaught AsseritionError

I'm getting this error from Mocha:

Uncaught AssertionError: expected 'long-title-abcdefghijklmnopqrs' to equal 'long-title-abcdefghijklmnopqrs'

This doesn't make any sense because those strings appear to be equal. Here's the test code:

it('shortens and joins title to 30 characters and with -', function(done){
    article.createMdArticle('long title abcdefghijklmnopqrstubwxyz', 'the bod', function(err, doc){
      if(err) throw err;
      doc.url_title.should.eql('long-title-abcdefghijklmnopqrs');
      done();
    })
})

and this mongoose pre('save') hook which creates url_title from title

articleSchema.pre('save', function (next) {
    //makes 'test title' into 'test-title'
    this.url_title = this.title.split(' ').join('-').substring(0,30);
    console.log(this.url_title);
    next();
});

All of my other tests that compare any other object data work as expected