subset expression not being accepted

I am trying to pass an expression to the subset(..) function as in:

```
#load "../packages/RProvider.1.1.12/RProvider.fsx"
//install UsingR first

open RDotNet
open RProvider
open RProvider.``base``
open RProvider.UsingR

//this does not work. Result is not a subset but the original dataset
let s = R.subset(R.father_son, "round(fheight) == 71")
```

The expression passed in has no effect on the output. The same expression works fine in R console.

Ordinarily one does not need to use subset() function as F# equivalent should work fine. However, in this case I am trying to convert some existing R code to F#. I want the same results as in R and then make more changes to make the code F# specific.