diff --git a/CHANGELOG.md b/CHANGELOG.md
index 68cf52b..367a4e6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- **The theme validator passes `formatVersion: null`.** lich decodes a null
+  `formatVersion` the way it decodes an omitted one and installs the theme;
+  the validator rejected it as not a positive integer.
+
 ## [0.18.3] - 2026-10-08
 
 ### Fixed
diff --git a/skills/theme/validate.mjs b/skills/theme/validate.mjs
index 3cee86e..9f5a652 100644
--- a/skills/theme/validate.mjs
+++ b/skills/theme/validate.mjs
@@ -40,9 +40,9 @@ if (!target) {
 }
 
 function formatVersionError(document) {
-  if (!("formatVersion" in document)) return null
   const version = document.formatVersion
-  // 0 is what lich decodes an omitted formatVersion to, so it reads as 1.
+  // lich decodes an omitted or null formatVersion to 0, and reads 0 as 1.
+  if (version === undefined || version === null) return null
   if (!Number.isInteger(version) || version < 0)
     return `theme formatVersion ${JSON.stringify(version)} must be a positive integer`
   if (version > FORMAT_VERSION)
diff --git a/tests/theme-validate.test.mjs b/tests/theme-validate.test.mjs
index 3ba0c5b..e1eb00d 100644
--- a/tests/theme-validate.test.mjs
+++ b/tests/theme-validate.test.mjs
@@ -56,6 +56,11 @@ test('formatVersion 0 passes, the way lich reads an omitted one', () => {
   assert.equal(result.status, 0, result.stderr)
 })
 
+test('formatVersion null passes, as decoding null into an int leaves lich at 0', () => {
+  const result = validateTheme({ formatVersion: null })
+  assert.equal(result.status, 0, result.stderr)
+})
+
 test('a negative formatVersion is rejected with lich\'s message', () => {
   const result = validateTheme({ formatVersion: -1 })
   assert.equal(result.status, 1)