title: R Markdown won't knit; Error in parse attempt to use zero-length variable name
body: <p>I'm trying to prepare an Rmarkdown file for a course. But for some reason (perhaps a very, very dumb reason) I can't seem to knit anything. Here is a very simple example of an Rmarkdown script:</p>
  
  <pre><code>    ---
      title: "big data"
      author: "CAM"
      date: "21 March 2020"
      output:
      html_document: default
      ---
  
      ```{r,chunk1,eval=TRUE,echo=TRUE,include=TRUE}
      seq(1,10,by=1)
      ```
  </code></pre>
  
  <p>This throws the error:</p>
  
  <pre><code>Error in parse(text = x, keep.source = TRUE) : 
    attempt to use zero-length variable name
  Calls: &lt;Anonymous&gt; -&gt; &lt;Anonymous&gt; -&gt; getParseData -&gt; parse
  Execution halted
  </code></pre>
  
  <p>The code itself executes in the chunk, but it won't knit. </p>
  
  <p>Here is my session info. (I just moved to catalina on mac, maybe there's a problem here).</p>
  
  <pre><code>R version 3.6.3 (2020-02-29)
  Platform: x86_64-apple-darwin15.6.0 (64-bit)
  Running under: macOS Catalina 10.15.3
  
  Matrix products: default
  BLAS:   /System/Library/Frameworks/Accelerate.framework/Versions/A/Frameworks/vecLib.framework/Versions/A/libBLAS.dylib
  LAPACK: /Library/Frameworks/R.framework/Versions/3.6/Resources/lib/libRlapack.dylib
  
  locale:
  [1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
  
  attached base packages:
  [1] stats     graphics  grDevices utils     datasets 
  [6] methods   base     
  
  other attached packages:
  [1] knitr_1.28    rmarkdown_2.1
  
  loaded via a namespace (and not attached):
  [1] compiler_3.6.3  htmltools_0.4.0 tools_3.6.3    
  [4] yaml_2.2.1      Rcpp_1.0.3      xfun_0.12      
  [7] digest_0.6.25   rlang_0.4.5     evaluate_0.14 
  </code></pre>