New Post: .When doesn't work

Hi, I am facing a strange problem when I write a rule to validate a field "document"

I coded this way.

RuleFor (model => model.DocumentId)
                 . NotEmpty (). WithLocalizedMessage (() => Resources.Customer.DocumentId)
                 . Length (1, 18). WithLocalizedMessage (() => Resources.Customer.DocumentIdLength)
                 . When (customer =>! Util.ValidateDocument (customer.DocumentId));

But for some reason .When is not working, could someone give me a help?

Thx