Don't crash when theme.keyframes is explicitly set to null

CHANGELOG.md

@@ -31,6 +31,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Canonicalization: don't merge utilities that reference different theme variables set to CSS-wide keywords like `unset` ([#20417](https://github.com/tailwindlabs/tailwindcss/pull/20417))
 - Don't generate utilities when a modifier is used that would otherwise be silently ignored (e.g. `rounded-sm/[5]`, `shadow-sm/foo`, `stroke-2/50`) ([#20419](https://github.com/tailwindlabs/tailwindcss/pull/20419))
 - Only normalize top-level `and`, `or`, and `not` keywords in `supports-[…]` variants (e.g. `selector(a: not (.foo))` → `selector(a:not(.foo))`) ([#20420](https://github.com/tailwindlabs/tailwindcss/pull/20420))
+- Don't crash when a JS config explicitly sets `theme.keyframes` to `null` ([#20425](https://github.com/tailwindlabs/tailwindcss/pull/20425))
 
 ## [4.3.3] - 2026-07-16
 

packages/tailwindcss/src/compat/apply-keyframes-to-theme.test.ts

@@ -55,6 +55,53 @@ test('keyframes can be merged into the theme', () => {
   `)
 })
 
+test('setting `theme.keyframes` to `null` does not throw and clears keyframes', () => {
+  let theme = new Theme()
+  let design = buildDesignSystem(theme)
+
+  let { resolvedConfig } = resolveConfig(design, [
+    {
+      config: {
+        theme: {
+          keyframes: null,
+        },
+      },
+      base: '/root',
+      reference: false,
+      src: undefined,
+    },
+  ])
+
+  expect(() => applyKeyframesToTheme(design, resolvedConfig)).not.toThrow()
+  expect(toCss(design.theme.getKeyframes())).toEqual('')
+})
+
+test('setting `theme.extend.keyframes` to `null` does not throw', () => {
+  // Unlike a top-level `theme.keyframes: null`, this is a no-op: `deepMerge`
+  // skips `null`/`undefined` sources entirely, so the resolved value here is
+  // `{}`, not `null`. This test guards against that assumption changing.
+  let theme = new Theme()
+  let design = buildDesignSystem(theme)
+
+  let { resolvedConfig } = resolveConfig(design, [
+    {
+      config: {
+        theme: {
+          extend: {
+            keyframes: null,
+          },
+        },
+      },
+      base: '/root',
+      reference: false,
+      src: undefined,
+    },
+  ])
+
+  expect(() => applyKeyframesToTheme(design, resolvedConfig)).not.toThrow()
+  expect(toCss(design.theme.getKeyframes())).toEqual('')
+})
+
 test('will append to the default keyframes with new keyframes', () => {
   let theme = new Theme()
   let design = buildDesignSystem(theme)

packages/tailwindcss/src/compat/apply-keyframes-to-theme.ts

@@ -14,7 +14,7 @@ export function applyKeyframesToTheme(
 
 export function keyframesToRules(resolvedConfig: Pick<ResolvedConfig, 'theme'>): AtRule[] {
   let rules: AtRule[] = []
-  if ('keyframes' in resolvedConfig.theme) {
+  if (resolvedConfig.theme.keyframes) {
     for (let [name, keyframe] of Object.entries(resolvedConfig.theme.keyframes)) {
       rules.push(atRule('@keyframes', name, objectToAst(keyframe as any)))
     }