diff --git a/lib/index.js b/lib/index.js
index ed48834..8b22c12 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -1,37 +1,60 @@
 /**
  * @typedef {import('nlcst').Root} Root
- * @typedef {import('nlcst').Word} Word
  *
+ * @typedef {import('vfile').VFile} VFile
+ */
+
+/**
  * @typedef Options
  *   Configuration.
- * @property {Array<string>} [ignore]
- *   Phrases *not* to warn about.
+ * @property {ReadonlyArray<string> | null | undefined} [ignore]
+ *   Phrases *not* to warn about (optional).
  */
 
 import {search} from 'nlcst-search'
 import {toString} from 'nlcst-to-string'
 import {findBefore} from 'unist-util-find-before'
-import {pointStart, pointEnd} from 'unist-util-position'
+import {pointEnd, pointStart} from 'unist-util-position'
 import {list} from './list.js'
 
-const source = 'retext-passive'
-const url = 'https://github.com/retextjs/retext-passive#readme'
+/** @type {Readonly<Options>} */
+const emptyOptions = {}
+/** @type {ReadonlyArray<never>} */
+const emptyIgnore = []
 
 const verbs = new Set(['am', 'are', 'were', 'being', 'is', 'been', 'was', 'be'])
 
 /**
- * Plugin to check for passive voice.
+ * Check for passive voice.
  *
- * @type {import('unified').Plugin<[Options?], Root>}
+ * @param {Readonly<Options> | null | undefined} [options]
+ *   Configuration (optional).
+ * @returns
+ *   Transform.
  */
-export default function retextPassive(options = {}) {
-  const ignore = options.ignore || []
+export default function retextPassive(options) {
+  const settings = options || emptyOptions
+  const ignore = settings.ignore || emptyIgnore
   const phrases =
-    ignore.length > 0 ? list.filter((d) => !ignore.includes(d)) : list
+    ignore.length > 0
+      ? list.filter(function (d) {
+          return !ignore.includes(d)
+        })
+      : [...list]
 
-  return (tree, file) => {
-    search(tree, phrases, (match, index, parent, phrase) => {
-      const before = /** @type {Word} */ (findBefore(parent, index, 'WordNode'))
+  /**
+   * Transform.
+   *
+   * @param {Root} tree
+   *   Tree.
+   * @param {VFile} file
+   *   File.
+   * @returns {undefined}
+   *   Nothing.
+   */
+  return function (tree, file) {
+    search(tree, phrases, function (match, index, parent, phrase) {
+      const before = findBefore(parent, index, 'WordNode')
 
       if (!before || !verbs.has(toString(before).toLowerCase())) {
         return
@@ -40,15 +63,16 @@ export default function retextPassive(options = {}) {
       const start = pointStart(match[0])
       const end = pointEnd(match[match.length - 1])
 
-      Object.assign(
-        file.message('Don’t use the passive voice', {
-          /* c8 ignore next -- hard to test */
-          place: start && end ? {start, end} : undefined,
-          source,
-          ruleId: phrase.replace(/\s+/g, '-').toLowerCase()
-        }),
-        {actual: toString(match), expected: [], url}
-      )
+      const message = file.message('Don’t use the passive voice', {
+        /* c8 ignore next -- hard to test */
+        place: start && end ? {start, end} : undefined,
+        source: 'retext-passive',
+        ruleId: phrase.replace(/\s+/g, '-').toLowerCase()
+      })
+
+      message.actual = toString(match)
+      message.expected = []
+      message.url = 'https://github.com/retextjs/retext-passive#readme'
     })
   }
 }
diff --git a/lib/list.js b/lib/list.js
index 58ab2de..8429516 100644
--- a/lib/list.js
+++ b/lib/list.js
@@ -1,3 +1,4 @@
+/** @type {ReadonlyArray<string>} */
 export const list = [
   'awoken',
   'awoke',
diff --git a/package.json b/package.json
index d443339..e9f3bea 100644
--- a/package.json
+++ b/package.json
@@ -35,9 +35,9 @@
     "@types/nlcst": "^2.0.0",
     "nlcst-search": "^4.0.0",
     "nlcst-to-string": "^4.0.0",
-    "unified": "^11.0.0",
     "unist-util-find-before": "^4.0.0",
-    "unist-util-position": "^5.0.0"
+    "unist-util-position": "^5.0.0",
+    "vfile": "^6.0.0"
   },
   "devDependencies": {
     "@types/node": "^20.0.0",
diff --git a/test.js b/test.js
index 63c298a..c92e2eb 100644
--- a/test.js
+++ b/test.js
@@ -4,6 +4,12 @@ import {retext} from 'retext'
 import retextPassive from './index.js'
 
 test('retext-passive', async function (t) {
+  await t.test('should expose the public api', async function () {
+    assert.deepEqual(Object.keys(await import('./index.js')).sort(), [
+      'default'
+    ])
+  })
+
   const doc = 'He was withheld while we were being fed.\nFed.\nThe fed.'
   const file = await retext().use(retextPassive).process(doc)