[Bug] Setting HTML mermaid-format to png leads to ERROR: AssertionError: with no message

[MurzNN]

I have:

• searched the issue tracker for similar issues
• installed the latest version of Quarto CLI
• formatted my issue following the Bug Reports guide

Bug description

I need to render HTML with Mermaid charts rendered as PNG images, not as SVG.
I tried to add an option to enable this mode like this:

format:
  html:
    mermaid-format: png

But after adding this, Quarto CLI starts to fail with no error message:

$ quarto preview /tmp/test.qmd
ERROR: AssertionError:
$

Steps to reproduce

1. Create a Quarto file like this:

---
format:
  html:
    mermaid-format: png
---
```{mermaid}
flowchart TD
  A --> B
```

2. Run Quarto CLI:

$ quarto preview /tmp/test.qmd

Actual behavior

It will output an error with no details:

ERROR: AssertionError: 

Expected behavior

It should render the chart in PNG, or, at least, display a meaningful error on what is done wrong.

Your environment

• IDE: VSCode
• OS: Ubuntu Linux

Quarto check output

Details

Quarto 1.9.37
[✓] Checking environment information...
      Quarto cache location: /home/alexey_korepov/.cache/quarto
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.8.3: OK
      Dart Sass version 1.87.0: OK
      Deno version 2.4.5: OK
      Typst version 0.14.2: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.9.37
      Path: /opt/quarto/bin

[✓] Checking tools....................OK
      Chrome Headless Shell: 148.0.7778.56
      TinyTeX: (not installed)
      Chromium: (not installed)
      VeraPDF: (not installed)

[✓] Checking LaTeX....................OK
      Tex:  (not detected)

[✓] Checking Chrome Headless....................OK
      Using: Chrome Headless Shell installed by Quarto
      Path: /home/alexey_korepov/.local/share/quarto/chrome-headless-shell/chrome-headless-shell-linux64/chrome-headless-shell
      Version: 148.0.7778.56

[✓] Checking basic markdown render....OK

[✓] Checking R installation...........(None)

      Unable to locate an installed version of R.
      Install R from https://cloud.r-project.org/

[✓] Checking Python 3 installation....OK
      Version: 3.13.5
      Path: /usr/bin/python3
      Jupyter: (None)

      Jupyter is not available in this Python installation.
      Install with python3 -m pip install jupyter

[✓] Checking Julia installation...

[mcanouil]

Could you follow the debugging log to get more details and paste the whole stack trace?

• https://quarto.org/docs/troubleshooting/index.html#basics

[MurzNN]

Thank you! Here is the debug output:

$ quarto preview /var/www/html/_docs/test.qmd --log-level debug
Quarto version: 1.9.37
[NotebookContext]: Starting Cleanup
[NotebookContext]: Starting Cleanup
Loaded deno-dom-native
[CHROMIUM] Using Chrome Headless Shell installed by Quarto: /home/alexey_korepov/.local/share/quarto/chrome-headless-shell/chrome-headless-shell-linux64/chrome-headless-shell
[CHROMIUM path]   : /home/alexey_korepov/.local/share/quarto/chrome-headless-shell/chrome-headless-shell-linux64/chrome-headless-shell
[CHROMIUM cmd]   : [object Object]
[NotebookContext]: Starting Cleanup
ERROR: AssertionError: 

But, seems not a lot of new details there, right?