@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Upgrade: Improve `pnpm` workspaces support ([#18065](https://.com/tailwindlabs/tailwindcss/pull/18065))
 - Upgrade: Migrate deprecated `order-none` to `order-0` ([#18126](https://.com/tailwindlabs/tailwindcss/pull/18126))
 - Support Leptos `class:` attributes when extracting classes ([#18093](https://.com/tailwindlabs/tailwindcss/pull/18093))
+- Fix "Cannot read properties of undefined" crash on malformed arbitrary value ([#18133](https://.com/tailwindlabs/tailwindcss/pull/18133))
 
 ## [4.1.7] - 2025-05-15
 

@@ -4305,11 +4305,11 @@ it("should error when `layer(…)` is used, but it's not the first param", async
 
 describe('`@reference "…" imports`', () => {
 test('recursively removes styles', async () => {
-let loadStylesheet = async (id: string, base: string) => {
+let loadStylesheet = async (id: string, base = '/root/foo') => {
 if (id === './foo/baz.css') {
 return {
+base,
 path: '',
-base: '/root/foo',
 content: css`
 .foo {
 color: red;
@@ -4355,11 +4355,11 @@ describe('`@reference "…" imports`', () => {
 })
 
 test('does not generate utilities', async () => {
-let loadStylesheet = async (id: string, base: string) => {
+let loadStylesheet = async (id: string, base = '/root/foo') => {
 if (id === './foo/baz.css') {
 return {
+base,
 path: '',
-base: '/root/foo',
 content: css`
 @layer utilities {
 @tailwind utilities;
@@ -4442,15 +4442,15 @@ describe('`@reference "…" imports`', () => {
 })
 matchUtilities(
 {
-'match-utility': (value) => ({
+'match-utility': (_value) => ({
 '@keyframes match-utilities': { '100%': { opacity: '0' } },
 }),
 },
 { values: { initial: 'initial' } },
 )
 matchComponents(
 {
-'match-components': (value) => ({
+'match-components': (_value) => ({
 '@keyframes match-components': { '100%': { opacity: '0' } },
 }),
 },
@@ -4474,30 +4474,30 @@ describe('`@reference "…" imports`', () => {
 })
 
 test('emits CSS variable fallback and keyframes defined inside @reference-ed files', async () => {
-let loadStylesheet = async (id: string, base: string) => {
+let loadStylesheet = async (id: string, base = '/root') => {
 switch (id) {
 case './one.css': {
 return {
+base,
 path: '',
-base: '/root',
 content: css`
 @import './two.css' layer(two);
 `,
 }
 }
 case './two.css': {
 return {
+base,
 path: '',
-base: '/root',
 content: css`
 @import './three.css' layer(three);
 `,
 }
 }
 case './three.css': {
 return {
+base,
 path: '',
-base: '/root',
 content: css`
 .foo {
 color: red;
@@ -5519,7 +5519,7 @@ describe('feature detection', () => {
 css`
 @import 'tailwindcss/preflight';
 `,
-{ loadStylesheet: async (_, base) => ({ base, content: '' }) },
+{ loadStylesheet: async (_, base) => ({ base, path: '', content: '' }) },
 )
 
 expect(compiler.features & Features.AtImport).toBeTruthy()
@@ -5530,7 +5530,7 @@ describe('feature detection', () => {
 css`
 @import 'tailwindcss/preflight';
 `,
-{ loadStylesheet: async (_, base) => ({ base, content: '' }) },
+{ loadStylesheet: async (_, base) => ({ base, path: '', content: '' }) },
 )
 
 // There's little difference between `@reference` and `@import` on a feature
@@ -5551,7 +5551,7 @@ describe('feature detection', () => {
 color: theme(--color-red);
 }
 `,
-{ loadStylesheet: async (_, base) => ({ base, content: '' }) },
+{ loadStylesheet: async (_, base) => ({ base, path: '', content: '' }) },
 )
 
 expect(compiler.features & Features.ThemeFunction).toBeTruthy()
@@ -5562,7 +5562,7 @@ describe('feature detection', () => {
 css`
 @plugin "./some-plugin.js";
 `,
-{ loadModule: async (_, base) => ({ base, module: () => {} }) },
+{ loadModule: async (_, base) => ({ base, path: '', module: () => {} }) },
 )
 
 expect(compiler.features & Features.JsPluginCompat).toBeTruthy()
@@ -5573,7 +5573,7 @@ describe('feature detection', () => {
 css`
 @config "./some-config.js";
 `,
-{ loadModule: async (_, base) => ({ base, module: {} }) },
+{ loadModule: async (_, base) => ({ base, path: '', module: {} }) },
 )
 
 expect(compiler.features & Features.JsPluginCompat).toBeTruthy()
@@ -5605,9 +5605,10 @@ describe('feature detection', () => {
 @reference "tailwindcss/utilities";
 `,
 {
-async loadStylesheet(id, base) {
+async loadStylesheet(_id, base) {
 return {
 base,
+path: '',
 content: css`
 @tailwind utilities;
 `,

@@ -165,6 +165,22 @@ describe('parse', () => {
 },
 ])
 })
+
+it('should not error when extra `)` was passed', () => {
+expect(parse('calc(1 + 2))')).toEqual([
+{
+kind: 'function',
+value: 'calc',
+nodes: [
+{ kind: 'word', value: '1' },
+{ kind: 'separator', value: ' ' },
+{ kind: 'word', value: '+' },
+{ kind: 'separator', value: ' ' },
+{ kind: 'word', value: '2' },
+],
+},
+])
+})
 })
 
 describe('toCss', () => {

@@ -302,7 +302,7 @@ export function parse(input: string) {
 // Handle everything before the closing paren a word
 if (buffer.length > 0) {
 let node = word(buffer)
-tail!.nodes.push(node)
+tail?.nodes.push(node)
 buffer = ''
 }