I have this very simple class..  

trigger RestrictContactByName on Contact (before insert, before update) {
    //check contacts prior to insert or update for invalid data
    For (Contact c : Trigger.New) {
        if(c.LastName == 'INVALIDNAME') {   //invalidname is invalid
            c.AddError('The Last Name "'+c.LastName+'" is not allowed for DML');
        }
    }
}

.. and the corresponding Test Class:  

@isTest
private class TestRestrictContactByName {

	@isTest static void metodoTest() {
		
		List listaContatti = new List();
		Contact c1 = new Contact(FirstName='Francesco', LastName='Riggio');
		Contact c2 = new Contact(LastName = 'INVALIDNAME');
		listaContatti.add(c1);
		listaContatti.add(c2);
		
		//insert listaContatti;
		
		// Perform test
        Test.startTest();
        Database.SaveResult [] result = Database.insert(listaContatti, false);
        Test.stopTest(); 
		
		c1.LastName = 'INVALIDNAME';
		update c1;
       		
	}
	
}

When I run the Test class from the Developer Console I get 100% of coverage on the RestrictContactByName class but, when I check the challenge on the trailhead it returns the error:

Challenge not yet complete... here's what's wrong: The 'RestrictContactByName' class did not achieve 100% code coverage via your test methods

Has someone had my same issue?