Svelte 5: TypeError: Cannot read properties of undefined (reading '__className')

[ichbtrv]

Describe the bug

A class with an empty string causes the app to crash after more than one element is added.

Reproduction

https://gist.github.com/ichbtrv/0d1bd92446506babf6f6430f60f6c9bc

Steps:

1. Comment out or delete the markup that is uncommented (lines 4 +5)
2. Uncomment the p or the h1 on lines 1 or 2, displaying only one element
3. Uncomment the remaining element from lines 1 or 2 to get the error.

Logs

Uncaught TypeError: Cannot read properties of undefined (reading '__className')

	in App.svelte

    at Module.set_class (chunk-ODCA3ZDS.js?v=3bd3c0ac:1588:29)
    at App (App.svelte?t=1725496964825:16:4)
    at chunk-ODCA3ZDS.js?v=3bd3c0ac:204:58
    at update_reaction (chunk-CDPR77WS.js?v=3bd3c0ac:1568:23)
    at update_effect (chunk-CDPR77WS.js?v=3bd3c0ac:1663:21)
    at create_effect (chunk-CDPR77WS.js?v=3bd3c0ac:991:7)
    at branch (chunk-CDPR77WS.js?v=3bd3c0ac:1120:10)
    at chunk-ODCA3ZDS.js?v=3bd3c0ac:198:17
    at update_reaction (chunk-CDPR77WS.js?v=3bd3c0ac:1568:23)
    at update_effect (:5173/node_modules/.vite/deps/chunk-CDPR77WS.js?v=3bd3c0ac:1663:21)TypeError: Cannot read properties of undefined (reading '__className')
    at Module.set_class (chunk-ODCA3ZDS.js?v=3bd3c0ac:1588:29)
    at App (App.svelte?t=1725496964825:16:4)
    at chunk-ODCA3ZDS.js?v=3bd3c0ac:204:58
    at update_reaction (chunk-CDPR77WS.js?v=3bd3c0ac:1568:23)
    at update_effect (chunk-CDPR77WS.js?v=3bd3c0ac:1663:21)
    at create_effect (chunk-CDPR77WS.js?v=3bd3c0ac:991:7)
    at branch (chunk-CDPR77WS.js?v=3bd3c0ac:1120:10)
    at chunk-ODCA3ZDS.js?v=3bd3c0ac:198:17
    at update_reaction (chunk-CDPR77WS.js?v=3bd3c0ac:1568:23)
    at update_effect (chunk-CDPR77WS.js?v=3bd3c0ac:1663:21)

System Info

System:
    OS: macOS 14.3
    CPU: (8) arm64 Apple M3
    Memory: 186.91 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.11.1 - ~/.nvm/versions/node/v20.11.1/bin/node
    Yarn: 1.22.19 - ~/Library/pnpm/yarn
    npm: 10.2.4 - ~/.nvm/versions/node/v20.11.1/bin/npm
    pnpm: 8.15.5 - ~/Library/pnpm/pnpm
  Browsers:
    Brave Browser: 128.1.69.153
    Chrome: 128.0.6613.119
    Safari: 17.3
  npmPackages:
    svelte: ^5.0.0-next.1 => 5.0.0-next.243,
    @sveltejs/vite-plugin-svelte: ^4.0.0-next.6

Severity

blocking all usage of svelte

[paoloricciuti]

This was already fixed by #13113 and would still be a duplicate of #13111 .

Here's a working repl

We will publish the fix in the next release. Thanks for reporting anyway 🙏🏻