diff --git a/lib/index.js b/lib/index.js
index ef01c0d..2b8f541 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -21,7 +21,7 @@
 /**
  * Get stats for a file, list of files, or list of messages.
  *
- * @param {VFile | VFileMessage | Array<VFile | VFileMessage> | null | undefined} [value]
+ * @param {VFile | VFileMessage | Array<VFile | VFileMessage>} value
  *   File, message, or list of files or messages.
  * @returns {Statistics}
  *   Statistics.
@@ -29,12 +29,16 @@
 export function statistics(value) {
   const result = {fatal: 0, warn: 0, info: 0}
 
-  if (value) {
-    if (Array.isArray(value)) {
-      list(value)
-    } else {
-      one(value)
-    }
+  if (!value) {
+    throw new TypeError(
+      'Expected file or message for `value`, not `' + value + '`'
+    )
+  }
+
+  if (Array.isArray(value)) {
+    list(value)
+  } else {
+    one(value)
   }
 
   return {
diff --git a/readme.md b/readme.md
index d0fdce3..4835aa4 100644
--- a/readme.md
+++ b/readme.md
@@ -94,7 +94,7 @@ Get stats for a file, list of files, or list of messages.
 ###### Parameters
 
 *   `file` ([`VFile`][vfile], [`VFileMessage`][vfile-message],
-    `Array<VFile | VFileMessage>`, optional)
+    `Array<VFile | VFileMessage>`)
     — file, message, or list of files or messages.
 
 ###### Returns
diff --git a/test.js b/test.js
index 4c437ac..a9b7d57 100644
--- a/test.js
+++ b/test.js
@@ -13,13 +13,10 @@ test('statistics', async function () {
   const file = new VFile()
   const other = new VFile()
 
-  assert.deepEqual(statistics(), {
-    fatal: 0,
-    nonfatal: 0,
-    warn: 0,
-    info: 0,
-    total: 0
-  })
+  assert.throws(function () {
+    // @ts-expect-error: check that a runtime error is thrown.
+    statistics()
+  }, /Expected file or message for `value`, not `undefined`/)
 
   assert.deepEqual(statistics(file), {
     fatal: 0,