title: rmarkdown error "attempt to use zero-length variable name"
body: <p>When i generate a new rmarkdown file (or open existing rmarkdown-files) and try to run a rmarkdown chunk, i get this error: "Error: attempt to use zero-length variable name".
  I have Win10 and did a fresh install of R and Rstudio yesterday. What did i miss? Where does this error come from?</p>
  
  <pre><code>```{r cars}
  summary(cars)
  ```
  </code></pre>
  
  <blockquote>
    <p>```{r cars}
    Error: attempt to use zero-length variable name</p>
  </blockquote>
  
  <p><a href="https://i.stack.imgur.com/FqebP.png" rel="noreferrer"><img src="https://i.stack.imgur.com/FqebP.png" alt="enter image description here"></a></p>