add volt function don't work

hi

why this code don't work???

    $volt->getCompiler()->addFunction('getBtCancelBack', function () {
        return  'hi';
    });

and calling in the volt file:

{{ getBtCancelBack() }}

I don't get the error message, the browser show only "the page cannot be shown". without php errors also.

php 5.5

phalcon 1.3.3 (tested on 1.3.2 also)

apache 2.4