Compiler cannot process conditions for strings that are null

[niden]

/**
 * Returns the inherited roles for a passed role name. If no role name
 * has been specified it will return the whole array. If the role has not
 * been found it returns an empty array
 */
public function getInheritedRoles(string roleName = null) -> array
{
    var result;

    if null !== roleName {
        return this->roleInherits;
    }

    if !fetch result, this->roleInherits[roleName] {
        return [];
    }

    return result;
}

This produces:

[ERROR] Unknown type: string in /srv/phalcon/Acl/Adapter/Memory.zep on line 527

If we change the condition:

if null !== roleName {

to

if !roleName {

or

if typeof roleName !== "string" {

it works fine.