Symfony2 Security Component And Propel, Incorrect table name in the select statement

Question

I read this page for manage my symfony2.0 login form with propel orm. On form submit propel returns this error:

Warning! Unable to execute SELECT statement 
[SELECT admin.ID, admin.USERNAME, admin.PASSWORD, admin.USERLEVEL, admin.MERCATO_ID 
FROM `` `admin` WHERE admin.USERNAME=:p1 LIMIT 1] 
[wrapped: SQLSTATE[42000]: Syntax error or access violation: 1103 Incorrect table name '']

It should be

FROM admin

and not

FROM `` `admin`

Why in the select statement there is `` before the table name admin ?

EDIT

security.yml content

security:
    providers:
        custom_provider:
            id: papernet.secured.security.provider
    role_hierarchy:
        ROLE_ADMIN:       ROLE_USER
        ROLE_SUPER_ADMIN: [ROLE_USER, ROLE_ADMIN, ROLE_ALLOWED_TO_SWITCH]
    firewalls:
        dev:
            pattern:  ^/(_(profiler|wdt)|css|images|js)/
            security: false
        login_firewall:
          pattern:    ^/login$
          anonymous:  ~
        secured_area:
            pattern:  ^/
            anonymous: ~
            form_login:
                check_path: /login_check
                login_path: /login
            logout:
                path:   /logout
                target: /login
            #anonymous: ~
            #http_basic:
            #    realm: "Secured Demo Area"

    access_control:
      - { path: ^/login, roles: IS_AUTHENTICATED_ANONYMOUSLY }
      - { path: ^/, roles: ROLE_ADMIN }

papernet.secured.security.provider is my custom provider as is shown in the guide