error with figure caption and markdown footnote when reference-location: margin and format: pdf

[dustinstoltz]

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

When rendering to a PDF, if reference-location is set to margin, a figure caption and a markdown-styled footnote are in the same document, it will error out. However, if instead of ^[Here is a marginnote], I use the latex version \footnote{Here is a marginnote with latex}, there is no error. If format is set to HTML, it renders fine.

Steps to reproduce

Error Example 1

---
title: Minimal Example
format:
    pdf:
      reference-location: margin
---

Here's some text.

```{r}
#| fig-cap: "Figure Label"

plot(cars)

```

Here's some more text.^[Here is a marginnote]

Error Example 2

---
title: Minimal Example
format:
    pdf:
      reference-location: margin
---

Here's some text.


![Elephant](elephant.png)

Here's some more text.^[Here is a marginnote]

Working Example 3

---
title: Minimal Example
format:
    pdf:
      reference-location: margin
---

Here's some text.

```{r}
#| fig-cap: "Figure Label"

plot(cars)

```

Here's some more text.\footnote{Here is a marginnote with latex}

Actual behavior

The qmd does not render to PDF, instead I get the following error: Error running filter /opt/quarto/share/filters/main.lua:
/opt/quarto/share/filters/main.lua:210: attempt to get length of a Block value (local 'blocks')

Expected behavior

No response

Your environment

• IDE: Positron 2025.09.0 build 139
• OS: Pop!_OS 24.04 LTS x86_64

Quarto check output

Quarto 1.8.25
[✓] Checking environment information...
      Quarto cache location: /home/dss/.cache/quarto
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.6.3: OK
      Dart Sass version 1.87.0: OK
      Deno version 2.3.1: OK
      Typst version 0.13.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.8.25
      Path: /opt/quarto/bin

[✓] Checking tools....................OK
      TinyTeX: (external install)
      Chromium: (not installed)

[✓] Checking LaTeX....................OK
      Using: TinyTex
      Path: /home/dss/.TinyTeX/bin/x86_64-linux
      Version: 2025

[✓] Checking Chrome Headless....................OK
      Chrome:  (not detected)

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

[✓] Checking Python 3 installation....OK
      Version: 3.10.14
      Path: /home/dss/.pyenv/versions/3.10.14/bin/python
      Jupyter: 5.7.1
      Kernels: python3

[✓] Checking Jupyter engine render....OK

[✓] Checking R installation...........OK
      Version: 4.3.3
      Path: /usr/lib/R
      LibPaths:
        - /home/dss/R/x86_64-pc-linux-gnu-library/4.3
        - /usr/local/lib/R/site-library
        - /usr/lib/R/site-library
        - /usr/lib/R/library
      knitr: 1.50
      rmarkdown: 2.30

[✓] Checking Knitr engine render......OK

[mcanouil]

The error only happens for non-figure float.
For instance, here you are a setting a caption, but you are not setting the image as a figure as you did not set any ID/Label.

The following document will render fine:

---
title: Minimal Example
format:
  pdf:
    reference-location: margin
---

Here's some text.

![Figure Label]({{< placeholder 600 400 >}}){#fig-placeholder}

Here's some more text.^[Here is a marginnote]

The issue is that an image with a caption is a Figure and figures are not treated as images.
In this case, the figure is missing the label/id which confuses the Lua filter in charge of dealing with figures.

[dustinstoltz]

Thanks so much for your quick response and explanation! Your example works for me. This works for me as well:

---
title: Minimal Example
format:
    pdf:
      reference-location: margin
---

Here's some text.

```{r}
#| label: fig-placeholder
#| fig-cap: "Figure Label"

plot(cars)

```

Here's some more text.^[Here is a marginnote]

And, I'll also note that I get the same error if the label is not prepended with fig- I see this noted in the documentation about making a figure cross-referenceable, but did not make the connection to this error message.

[mcanouil]

The fix for this is actually a very simple one.
I'll push a PR and check the CI tests suite in case it has unforeseen side-effects.