Negate expression to put in "disabled" property

If I put ${security.user.isInRole('am')} expression in disabled property of a checkbox it works perfectly.
Unfortunately I want the opposite: if the user is in role "am" the checkbox should be not disabled.
No matter what I do (putting ! using == and so on) the negation does not work, it is always disabled.
Probably I am missing something really stupid.
Can you help me?

Hi @mgiammarco,

Does ${security.isInRole('am') == false} work? Can you check what the generated code is?