Deselecting multpile checkboxes don't run validation.

[webskydavid]

Hi.
I have multiple checkboxes, if I press submit it show me validation errors. After pressing one of them the validation error disapperars.
The values if one checkbox is pressed looks like this:
["30"]
After deselecting all checkboxes looks like this:
[] <- thats wrong, the value need to be empty.
The validation works only if I add onSet function to binding:

'input.usr:checkbox': {
      observe   : 'usr',
      onSet: function (value) {
        if(value.length > 0){
          return value;
        }
        return '';
      },
      setOptions: {
        validate: true
      }
    }

Example: http://jsfiddle.net/c3kK2/474/