FactoryMuffin in Laravel 5 app does not create test data with arrays for mongoDB

I have a laravel 5 app that uses MongoDB as its secondary data store and unit tests set up that makes use of FactoryMuffin.

One of the collections/models has an embedded doc structure:

{
    details: {title:'first name', other:'other name'}
}

To get started I followed the FactoryMuffin Tutorial.

Factory Definition:

FactoryMuffin::define(
'App\User',
array(
    'details'=>array(
         'title'=>'first name', 
         'other'=>'other_name'
        )
)
);

Actual Test:

public function testSampleFactory()
{

    $user = FactoryMuffin::create('App\User');
    $this->assertInstanceOf('App\User', $user);
}

But I came across an error:

There was 1 error:

1) TestUserModel::testSampleFactory
    substr() expects parameter 1 to be string, array given

/var/www/wfd2/app/vendor/league/factory-muffin/src/Generators/Base.php:87
/var/www/wfd2/app/vendor/league/factory-muffin/src/Factory.php:660
/var/www/wfd2/app/vendor/league/factory-muffin/src/Factory.php:600
/var/www/path/app/vendor/league/factory-muffin/src/Factory.php:348
/var/www/path/app/vendor/league/factory-muffin/src/Factory.php:259
/var/www/path/app/vendor/league/factory-muffin/src/Facade.php:93
/var/www/path/app/tests/ExampleTest.php:17
/var/www/path/app/tests/ExampleTest.php:17  

It seems FactoryMuffin is expecting attribute values to be strings only. Is this the case? How would I add embedded/nested test data with Factory Muffin?

Thank you