Validate a number, which depends on other validate numbers

I want to validate a number, which depends on other values. I tried something like the following:

``` js
var schema = {
  a: Joi.number().
  b: Joi.number()

  c: Joi.number().max(Joi.ref('a') + Joi.ref('b'))
};
```

In this case c should not be greater than the value of a + b. 

If I run this code, I get 

``` js
Error: limit must be a number or reference
```
