diff --git a/package.json b/package.json
index 52777cde3c..67af57da5c 100644
--- a/package.json
+++ b/package.json
@@ -24,7 +24,7 @@
 		"smoketest-ui": "playwright test --config=test/smoke/playwright.config.ts --ui",
 		"smoketest-headed": "playwright test --config=test/smoke/playwright.config.ts --headed",
 		"test": "ts-node ./build/check-samples && npm run test:grammars",
-		"test:grammars": "node --import tsx --import ./test/test-setup.mjs --test \"src/languages/definitions/*/*.test.ts\"",
+		"test:grammars": "node --import tsx --import ./test/test-setup.mjs --test \"src/languages/**/*.test.ts\"",
 		"deps-all-remove": "ts-node ./build/npm/removeAll",
 		"deps-all-install": "ts-node ./build/npm/installAll",
 		"update-actions": "pin-github-action ./.github/workflows/website.yml",
diff --git a/src/languages/features/css/cssWorker.test.ts b/src/languages/features/css/cssWorker.test.ts
new file mode 100644
index 0000000000..ebab90799b
--- /dev/null
+++ b/src/languages/features/css/cssWorker.test.ts
@@ -0,0 +1,31 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+import { describe, it } from 'node:test';
+import assert from 'node:assert';
+import { CSSWorker } from './cssWorker';
+
+function createWorker(content: string, uri = 'file:///test.css'): CSSWorker {
+	const model = {
+		uri: { toString: () => uri },
+		version: 1,
+		getValue: () => content
+	};
+	const ctx = { getMirrorModels: () => [model] } as any;
+	return new CSSWorker(ctx, { languageId: 'css', options: {} });
+}
+
+describe('CSSWorker', () => {
+	it('doHover does not throw on invalid nth-child selectors', async () => {
+		const worker = createWorker(':nth-child() { }');
+		assert.strictEqual(await worker.doHover('file:///test.css', { line: 0, character: 3 }), null);
+	});
+
+	it('doHover returns a hover for valid selectors', async () => {
+		const worker = createWorker('li:nth-child(2n of .important) { }');
+		const hover = await worker.doHover('file:///test.css', { line: 0, character: 1 });
+		assert.ok(hover);
+	});
+});
diff --git a/src/languages/features/css/cssWorker.ts b/src/languages/features/css/cssWorker.ts
index ced922370a..d912a341de 100644
--- a/src/languages/features/css/cssWorker.ts
+++ b/src/languages/features/css/cssWorker.ts
@@ -79,8 +79,14 @@ export class CSSWorker {
 			return null;
 		}
 		const stylesheet = this._languageService.parseStylesheet(document);
-		const hover = this._languageService.doHover(document, position, stylesheet);
-		return Promise.resolve(hover);
+		try {
+			const hover = this._languageService.doHover(document, position, stylesheet);
+			return Promise.resolve(hover);
+		} catch (e) {
+			// The CSS language service can throw on invalid selectors, e.g. `:nth-child()`.
+			// https://github.com/microsoft/monaco-editor/issues/5430
+			return null;
+		}
 	}
 	async findDefinition(
 		uri: string,