Controller helper Form validate('groupname') error with rules group

[luistrigo]

---

name: Bug report
about: Controller helper

---

Describe the bug
From Documentation:
Docs » Controllers and Routing » Controllers
Validating $_POST data

if (! $this->validate('signup'))

I get this error:
Argument 1 passed to CodeIgniter\Validation\Validation::setRules() must be of the type array, string given

Config\Validation
public $signup = [
'username' => 'required',
'password' => 'required',
'pass_confirm' => 'required|matches[password]',
'email' => 'required|valid_email'
];
public $signup_errors = [
'username' => [
'required' => 'You must choose a username.',
],
'email' => [
'valid_email' => 'Please check the Email field. It does not appear to be valid.'
]
];

CodeIgniter 4 version
CodeIgniter 4.0.0 alpha.2

Context

• OS: [e.g. MAC OS 10.14]
• Web server [e.g. Apache]
• PHP version [e.g. 7.2.8]