PhpUnit giving errror at second test function

I am trying to make test to my Database with phpunit and I migrate the database to memory.

the first test run just fine:

  /** @test */
     public function it_fetches_a_single_ano_letivo()
     {
         $this->makeAnoLetivo();

         $this->getJson('/v1/anos-letivos');

         $this->assertResponseOk();
     }

but the second test fails and its exactly the same as the first one:

     /** @test */
         public function it_fetches_anos_letivos()
         {
             $this->makeAnoLetivo();

             $this->getJson('/v1/anos-letivos');

             $this->assertResponseOk();
         }

Here is the makeAnoLetivo function:

 private function makeAnoLetivo($anoLetivoFields = [])
     {
         while($this->times--)
         {
             $ano1=$this->fake->year;

             $anoLetivo = array_merge([
                 'ano1' => $ano1+0,
                 'ano2' => $ano1+1
             ], $anoLetivoFields);

             AnoLetivo::create($anoLetivo);

         }
      }

and here is the phpUnit output:

Configuration read from {{PATH_TO_PROJECT}}/phpunit.xml

..E

Time: 2.62 seconds, Memory: 23.25Mb

There was 1 error:

1) AnosLetivosTest::it_fetches_anos_letivos
Illuminate\Database\QueryException: SQLSTATE[23000]: Integrity constraint violation: 19 anos_letivos.id may not be NULL (SQL: insert into "anos_letivos" ("ano1", "ano2", "updated_at", "created_at") values (2009, 2010, 2015-03-27 18:41:59, 2015-03-27 18:41:59))

{{PATH_TO_PROJECT}}/vendor/laravel/framework/src/Illuminate/Database/Connection.php:620
{{PATH_TO_PROJECT}}/vendor/laravel/framework/src/Illuminate/Database/Connection.php:576
{{PATH_TO_PROJECT}}/vendor/laravel/framework/src/Illuminate/Database/Connection.php:359
{{PATH_TO_PROJECT}}/vendor/laravel/framework/src/Illuminate/Database/Connection.php:316
{{PATH_TO_PROJECT}}/vendor/laravel/framework/src/Illuminate/Database/Query/Builder.php:1702
{{PATH_TO_PROJECT}}/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Builder.php:933
{{PATH_TO_PROJECT}}/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Model.php:1603
{{PATH_TO_PROJECT}}/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Model.php:1603
{{PATH_TO_PROJECT}}/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Model.php:1501
{{PATH_TO_PROJECT}}/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Model.php:544
{{PATH_TO_PROJECT}}/tests/AnosLetivosTest.php:50
{{PATH_TO_PROJECT}}/tests/AnosLetivosTest.php:32
phar:///usr/local/bin/phpunit/phpunit/TextUI/Command.php:152
phar:///usr/local/bin/phpunit/phpunit/TextUI/Command.php:104

Caused by
PDOException: SQLSTATE[23000]: Integrity constraint violation: 19 anos_letivos.id may not be NULL

{{PATH_TO_PROJECT}}/vendor/laravel/framework/src/Illuminate/Database/Connection.php:358
{{PATH_TO_PROJECT}}/vendor/laravel/framework/src/Illuminate/Database/Connection.php:612
{{PATH_TO_PROJECT}}/vendor/laravel/framework/src/Illuminate/Database/Connection.php:576
{{PATH_TO_PROJECT}}/vendor/laravel/framework/src/Illuminate/Database/Connection.php:359
{{PATH_TO_PROJECT}}/vendor/laravel/framework/src/Illuminate/Database/Connection.php:316
{{PATH_TO_PROJECT}}/vendor/laravel/framework/src/Illuminate/Database/Query/Builder.php:1702
{{PATH_TO_PROJECT}}/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Builder.php:933
{{PATH_TO_PROJECT}}/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Model.php:1603
{{PATH_TO_PROJECT}}/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Model.php:1603
{{PATH_TO_PROJECT}}/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Model.php:1501
{{PATH_TO_PROJECT}}/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Model.php:544
{{PATH_TO_PROJECT}}/tests/AnosLetivosTest.php:50
{{PATH_TO_PROJECT}}/tests/AnosLetivosTest.php:32
phar:///usr/local/bin/phpunit/phpunit/TextUI/Command.php:152
phar:///usr/local/bin/phpunit/phpunit/TextUI/Command.php:104

FAILURES!                          
Tests: 3, Assertions: 5, Errors: 1.

So the first function run just fine but the second one is the same and fails... Also if i create a third one (equal) only the first one will pass.