No content renderer found for markup "markdown" error

I’m developing an annotation shortcode, and I’m running into a problem with the RenderString function. I’ve put my shortcode code and an example in this gist Hugo annotation shortcode · GitHub

The general idea is that this

{{< annotate >}}
this is a codeblock with annotation # (1)
* this is a list (2) with annotation

  {{< annotations >}}
  1. An annotation :wave:
  2. Another annotation :dog:
  {{< /annotations >}}
{{< /annotate >}}

Will render this

This process is this:

  1. Markdownify the annotate .Inner, and capture to a string of HTML
  2. annotations creates script tags, where the script adds annotations elements and open/closes popups
  3. Find/replace the annotation anchor (\d) with a span/icon clickable element
  4. Render the full string of HTML (the .Inner content + anchors + script tags) to the doc

I think {{ $.Page.RenderString (dict "markup" "html") }} should render the whole string as HTML, but when I try that, I get the following error:

No content renderer found for markup "markdown"

Interesting, when I try (dict "markup" "pandoc"), I get the warning:

pandoc not found in $PATH: Please install.
                  Leaving pandoc content unrendered.

but the HTML ends up getting rendered correctly.

Am I using (dict "markup" "html") correctly? Why is this error being thrown, since I’m just trying to use HTML, which doesn’t have dependencies?