"Entry point ... cannot be marked as external" error when viewing bundle size for @sveltejs/package built libraries

[techniq]

Problem

When attempted to inspect Svelte UX on pkg-size.dev, I receive the following error:

Error: Process exited with code 1
✘ [ERROR] The entry point "svelte-ux" cannot be marked as external

Error: Build failed with 1 error:
error: The entry point "svelte-ux" cannot be marked as external
    at _0x389c10._evaluate (https://httpspkgsizedev-jeqp.w-corp.staticblitz.com/blitz.420f2654.js:352:382041)
    at async ModuleJob.run (https://httpspkgsizedev-jeqp.w-corp.staticblitz.com/blitz.420f2654.js:181:2372) {
  errors: [
    {
      detail: [90mundefined[39m,
      id: [32m''[39m,
      location: [1mnull[22m,
      notes: [],
      pluginName: [32m''[39m,
      text: [32m'The entry point "svelte-ux" cannot be marked as external'[39m
    }
  ],
  warnings: []
}

Svelte UX is a large Svelte component/actions/etc library built using @sveltejs/package. My other package, LayerChart, has the same issue. Also just tested Melt-UI, and it same the same issue too

Expected behavior

Able to view bundle size for a @sveltejs/package

Minimal reproduction URL

https://pkg-size.dev/svelte-ux

Version

N/A

Node.js version

N/A

Package manager

npm

Operating system

macOS

Contributions

• [ ] I plan to open a pull request for this issue
• [ ] I plan to make a financial contribution to this project