radian: Error when trying to use dplyr::mutate() in an RMarkdown (Rmd) file

I am running:

• Ubuntu 16.04.5 LTS on an x86_64 platform,
• R 3.5.2 with dplyr 0.8.0.1 and tibble 2.0.1, and
• radian 0.3.1 on Python 3.7.2.

The following RMarkdown (Rmd) file…

---
title: "Minimal working example"
output:
  pdf_document:
    latex_engine: pdflatex
---

```{r}
library(tibble)
library(dplyr)

tbl = tribble(
  ~category, ~value,
  "a", 1,
  "a", 2,
  "b", 3,
  "b", 4)

tbl %>% mutate(category = toupper(category))
```

…does not compile when I attempt to render it from a radian session:

R version 3.5.2 (2018-12-20) -- "Eggshell Igloo"
Platform: x86_64-pc-linux-gnu (64-bit)

r$> rmarkdown::render("tmp.Rmd")                                                              


processing file: tmp.Rmd
  |................................                                 |  50%
  ordinary text without R code

  |.................................................................| 100%
label: unnamed-chunk-1
Quitting from lines 9-20 (tmp.Rmd) 
Error in mutate_impl(.data, dots, caller_env()) : 
  SET_VECTOR_ELT() can only be applied to a 'list', not a 'symbol'

The above function (rmarkdown::render("tmp.Rmd")) does, however, work when in an R session.

Do you have any ideas about what the issue is?