Wrong signature when there is no commands

The signature is wrong when we try to create an URL without any commands, for proxying reasons for example:

$thumbnailUrlFactory = Thumbor\Url\BuilderFactory::construct('https://DOMAINE', $secret);

echo $thumbnailUrlFactory
    ->url('http://images.example.com/llamas.jpg')
    ->build()
    ->toString();

Result in:
https://DOMAINE/w_ENl4PO7AIjS4x33fmoAQDI-kQ=//http://images.example.com/llamas.jpg

which is wrong and must be:

https://DOMAINE/I5ejYVaMsVMGWaxaVKrgSw3UoP0=/http://images.example.com/llamas.jpg