Using purl to source a .Rmd file from within a Rnw file

I have done some analysis in a .Rmd file. I now want to use some objects that are created in this file in a report that I'm writing up as a .Rnw file. Since switching from Sweave to knittr as the weaving engine the following happens:

If i run the line purl(input = 'myfile.Rmd', output = 'myfile.R') in the console, I get an .R file that only contains the R chunks from the .Rmd file. That is what I want. If I put this line into the .Rnw file and knit it (the .Rnw file, that is) however, I do end up with a myfile.R and no errors, but it is completely empty (except for one linebreak for some reason).

I have also tried to put knitr::opts_chunk$set(purl = TRUE) and knit_hooks$set(purl = hook_purl) in the .Rmd file and then use knit() instead of purl() in my .Rnw file, but the result is the same.

The following is a small example:

test.Rnw

\documentclass{article}

\begin{document}

<<test>>=
  library(knitr)
  purl(input = 'test.Rmd', output = 'test.R')
@

\end{document}

test.Rmd

```{r}
answer <- 42
``` #

(Ignore the #, I didn't know how else to make the ticks show up)

Expected Output:

## ------------------------------------------------------------------------
answer <- 42

Actual output:

Does anyone have experience with this? Is this a bug or am I missing something? Thank you for your help!