Add a Function Callback to Rules Validation Jquery

Question

When I have this in my validation rules, it's working on,

'my_field': {
    required: true,
    number: false,
 }

But when I tried to do this,

'my_field': {
    required: true,
    number: function(){
      return false
    },
}

This always set my number to true, Why not false?

What did I do wrong?