I am trying to add missing data to my time series with the `$densify` operator. 

But it does not seem to work when I specify any dynamic values in the bounds array.

*Example*
With a simple data set:
```js
[
   { week: 1, value: 20 },
   { week: 2, value: 30 },
]
```
I want to fill in all weeks *up until todays date* 

So with the following `$densify` operator I would expect it to work properly:
```js
{
  field: "week",
  range:

{     step: 1,     bounds: [       0,       \{ $week: "$$CLUSTER_TIME" }

    ]
  }
}
```

But I get the following error:

`A bounding array must contain either both dates or both numeric types`

Which does not make sense, because `{ $week: "$$CLUSTER_TIME" }` evalutes to a number, and 0 is definetly a number last I checked.

Sounds like a bug with mongo to me, anyone know?