Zend Form ->addValidator('Digits') not working

I am trying to add validator to my Text field as i am expecting user to only enter positive integers. But its not working neither showing any error message if we enter value like '-50'. The code is, $AddInventory = new Zend_Form_Element_Text('AddInventory'); $AddInventory->setAttrib('dojoType',"dijit.form.ValidationTextBox");
$AddInventory->setAttrib('maxlength','40') ->setAttrib('style','width:200px') ->setAttrib('required',"false") ->addValidator('NotEmpty') ->addFilter('StripTags') ->addValidator('Digits')
->removeDecorator("DtDdWrapper") ->removeDecorator("Label") ->removeDecorator('HtmlTag');