php - Intermittent SaveFailedException on FactoryMuffin Confide user creation -

for laravel project using confide package i've tried create factory muffin (v2.1) definition creating users in integration tests.

factorymuffin::define('user', [ 'username'      => 'randomnumber|6', 'email'         => 'unique:email', 'password'      => function() { return 'test123'; }, 'password_confirmation' => function() { return 'test123'; }, 'confirmed'     => function() { return 1; }, 'confirmation_code' => function() { return md5(microtime().config::get('app.key')); } 

]);

however, during testing following error, of time.

league\factorymuffin\exceptions\savefailedexception: not save model of type: 'user'.

i've looked source of exception , tried printing model creation errors, these appear empty.

i've looked @ confide's uservalidator, far can see, generated user should validate.

has encountered problem before, or see have overlooked?