R Markdown - Error message because no answer can be given for prompt

I am in the process of writing my first vignette using R Markdown and I have hit a stumbling block when putting pieces of code into a chunk. The code is a function which, when run in R, will prompt the user for a numeric value. This can't be achieved in R Markdown so it throws an error message:

Error in if (h < 1.1) { : missing value where TRUE/FALSE needed calls: ... withCallHandlers -> withVisible -> eval -> eval -> heightvalue

I know that if I run:

```{r error = TRUE} 

it will allow the markdown to run and put the error message in my output (not ideal!).

I can get rid of this by putting:

 ```{r error = TRUE, eval = FALSE} 

but this means that all of my output disappears, i.e. no prompt will be displayed in my HTML. I would like the output to remain but the error message to disappear please.

If anyone can help, that'd be fantastic.

The original piece of code, which I call with heightvalue(h), is:

heightvalue <- function(h){
    h <- ask("Enter height for variable threshold (between 1.1 and 10): ")
    h <- (as.numeric(h))
    if(h < 1.1){
        stop('VALUE OUT OF BOUNDS')
    } else if(h > 10){
        stop('VALUE OUT OF BOUNDS')
    }
    return((h))
}