# Syntax highlighting causes codemirror plugin to crash if only not on a specific version of @lezer/highlight

**URL:** <https://discuss.codemirror.net/t/syntax-highlighting-causes-codemirror-plugin-to-crash-if-only-not-on-a-specific-version-of-lezer-highlight/9638>\
**Category:** v6\
**Created:** [January 4, 2026, 1:51am UTC](https://discuss.codemirror.net/t/syntax-highlighting-causes-codemirror-plugin-to-crash-if-only-not-on-a-specific-version-of-lezer-highlight/9638 "2026-01-04T01:51:29Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![ezntek](https://discuss.codemirror.net/user_avatar/discuss.codemirror.net/ezntek/32/5377_2.png) [@ezntek](https://discuss.codemirror.net/u/ezntek)\
**Post date:** [January 4, 2026, 1:51am UTC](https://discuss.codemirror.net/t/syntax-highlighting-causes-codemirror-plugin-to-crash-if-only-not-on-a-specific-version-of-lezer-highlight/9638/1 "2026-01-04T01:51:30Z")

</div>

When I try to use any language plugin with CodeMirror (let’s say, Python), and load it in EditorState.create, no syntax highlighting actually happens. The plugin in the console simply says:

`CodeMirror plugin crashed: TypeError: can't access property Symbol.iterator, tags is undefined`

Sometimes, it says `tags3` is undefined.

I have already checked the other posts and on other internet forums where people have had this issue. Their fixes of downgrading to `"@lezer/highlight": "1.1.6",` doesnt actually work on bun.js but only node. I am using vite, I found a solution whereby someone added everything CodeMirror/Lezer related to `optimizeDeps.exclude` which still doesn’t work. With `@lezer/highlight` on the latest version, nothing works, therefore the only way in which I can get this module to not crash is to do the following:

1. use `@lezer/highlight` version . 1.1.6
2. When installing dependencies, you cannot use bun.js at all. you must run `npm install`and then when you run it, you must run `npx vite dev` and only then does `bun run dev`/`bunx vite dev` lead to the plugin not crashing.

If any of those requirements are not met, it gives this stack trace:

```auto

CodeMirror plugin crashed: TypeError: can’t access property Symbol.iterator, tags3 is undefined
style index.js:284
highlightTags index.js:301
highlightRange index.js:388
highlightTree index.js:329
buildDeco index.js:1784
update index.js:1775
update index.js:1465
updatePlugins index.js:7838
update index.js:7736
dispatchTransactions index.js:7638
dispatch index.js:7662
Editor Editor.svelte:158
update_reaction runtime.js:264
update_effect runtime.js:439
flush_queued_effects batch.js:699
process batch.js:195
flush_effects batch.js:642
flush batch.js:345
ensure batch.js:523
run_all utils.js:45
run_micro_tasks task.js:10
queue_micro_task task.js:28
queue_micro_task task.js:19
enqueue batch.js:533
ensure batch.js:517
internal_set sources.js:187
set sources.js:166
set proxy.js:303
changeFile editor_state.svelte.ts:41
fileResponseCallback EditorWindow.svelte:146
handleWorkerEvent handle_worker.svelte.ts:57
setupWorker handle_worker.svelte.ts:94
EditorWindow EditorWindow.svelte:79
untrack runtime.js:715
onMount index-client.js:100
update_reaction runtime.js:264
update_effect runtime.js:439
flush_queued_effects batch.js:699
process batch.js:195
flush_effects batch.js:642
flush batch.js:345
ensure batch.js:523
run_all utils.js:45
run_micro_tasks task.js:10
flush_tasks task.js:40
flushSync batch.js:579
Svelte4Component legacy-client.js:127
 legacy-client.js:54
initialize client.js:587
_hydrate client.js:2855
start client.js:361
async* editor:1963
promise callback* editor:1962
index.js:1354:17

```

I’m using SvelteKit with Svelte 5, if that helps.

The dependencies section of my package.json is as follows:

```auto
	"dependencies": {
		"@battlefieldduck/xterm-svelte": "^2.2.0",
		"@catppuccin/palette": "^1.7.1",
		"@codemirror/autocomplete": "^6.20.0",
		"@codemirror/lang-python": "^6.2.1",
		"@codemirror/language": "^6.12.1",
		"@codemirror/lint": "^6.9.2",
		"@codemirror/theme-one-dark": "^6.1.3",
		"@fontsource/ibm-plex-mono": "^5.2.7",
		"@fontsource/inter": "^5.2.8",
		"@lezer/generator": "^1.8.0",
		"@lezer/highlight": "1.1.6",
		"@lezer/lr": "^1.4.5",
		"@xterm/xterm": "^6.0.0",
		"codemirror": "6",
		"jszip": "^3.10.1",
		"typescript-svelte-plugin": "^0.3.50"
	}

```

Why does this strange error happen only with the latest version? This happens for any and every grammar and language support extensoin, including the official ones.

---

<div class="post-metadata">

**Author:** ![marijn](https://discuss.codemirror.net/user_avatar/discuss.codemirror.net/marijn/32/136_2.png) [@marijn](https://discuss.codemirror.net/u/marijn)\
**Post date:** [January 4, 2026, 4:55pm UTC](https://discuss.codemirror.net/t/syntax-highlighting-causes-codemirror-plugin-to-crash-if-only-not-on-a-specific-version-of-lezer-highlight/9638/2 "2026-01-04T16:55:31Z")

</div>

Did you try clearing your package lock and reinstalling? Whatever tool you’re using to install dependencies is, by the sound of it, installing multiple versions of packages.