diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
index 0000000..4bc4cd4
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,66 @@
+# Changelog
+
+All notable changes to this project are documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Added
+
+- Non-string labels render instead of disappearing. Any value that isn't an array is stringified, at any depth, and can be a parent node: `treeify(['deploys', [2025, ['spring']]])` renders `2025` with a child.
+- `LICENSE` file (MIT). The README had linked to it since the beginning, but it was never committed. It now ships in the npm tarball.
+- `exports`, `sideEffects` and `engines` fields in `package.json`.
+- Continuous integration on GitHub Actions: tests and build across Node.js 22, 24 and 26.
+- This changelog.
+
+### Changed
+
+- **Breaking:** input validation now throws a `TypeError` rather than an `Error`, with a message naming what it received: `array-treeify: expected the first element to be a string, received number (1)`. It used to be `Error: First element must be a string`.
+- **Breaking:** input that is not an array now throws instead of returning an empty string. `treeify(null)` used to render nothing at all, which quietly hid the mistake.
+- **Breaking:** `treeify([undefined])` now throws. It is the same failure as any other non-string first element, and used to return an empty string.
+- **Breaking:** requires Node.js 22 or newer. Node.js 20 reached end of life on 2026-04-30.
+- `treeify` accepts numbers, bigints and booleans as labels without a cast.
+- Development toolchain: Biome 2 and TypeScript 7. `tsx` is gone — tests are `.ts` files run directly by `node --test` using Node's built-in type stripping, and `npm run typecheck` now covers the test files, which it never did before.
+
+`treeify([])` still returns an empty string.
+
+## [0.1.5] - 2025-05-08
+
+### Changed
+
+- More permissive input type.
+
+## [0.1.4] - 2025-05-07
+
+### Changed
+
+- `TreeInput` accepts an empty array.
+- Upgraded dependencies.
+
+## [0.1.3] - 2025-03-30
+
+### Added
+
+- `chars` option for custom tree characters.
+- `plain` option for whitespace-only output.
+
+## [0.1.2] - 2025-03-29
+
+### Changed
+
+- Tests are no longer published to npm.
+
+## [0.1.1] - 2025-03-28
+
+### Added
+
+- Initial release.
+
+[Unreleased]: https://github.com/tbeseda/array-treeify/compare/0.1.5...HEAD
+[0.1.5]: https://github.com/tbeseda/array-treeify/compare/0.1.4...0.1.5
+[0.1.4]: https://github.com/tbeseda/array-treeify/compare/0.1.3...0.1.4
+[0.1.3]: https://github.com/tbeseda/array-treeify/compare/0.1.2...0.1.3
+[0.1.2]: https://github.com/tbeseda/array-treeify/compare/0.1.1...0.1.2
+[0.1.1]: https://github.com/tbeseda/array-treeify/releases/tag/0.1.1
diff --git a/README.md b/README.md
index dde4168..702bf0b 100644
--- a/README.md
+++ b/README.md
@@ -152,13 +152,10 @@ Lumon Industries
 
 ## Input Format
 
-> **Disclaimer:**
-> The exported `TreeInput` type (`Array<string | TreeInput>`) is intentionally flexible to support dynamic and programmatic tree construction. However, TypeScript cannot enforce at the type level that the first element is a string. This requirement is checked at runtime by the `treeify` function, which will throw an error if the first element is not a string. Please ensure your input arrays follow this convention.
-
 The `treeify` function accepts arrays with the following structure:
 
 1. First element must be a string (the root node)
-2. Subsequent elements can be strings (nodes at same level) or arrays (children of previous node)
+2. Subsequent elements can be labels (nodes at same level) or arrays (children of previous node)
 3. Arrays can be nested to any depth
 
 ```typescript
@@ -167,6 +164,35 @@ The `treeify` function accepts arrays with the following structure:
 ['root', ['child', ['grandchild']]]                   // Grandchildren
 ```
 
+### Labels
+
+Any value that isn't an array is a label. Non-strings are stringified, so numbers and booleans work anywhere a string does — including as parent nodes:
+
+```typescript
+console.log(treeify(['deploys', [2025, ['spring', 'summer'], 2026, ['q1']]]))
+/*
+deploys
+├─ 2025
+│  ├─ spring
+│  └─ summer
+└─ 2026
+   └─ q1
+*/
+```
+
+### Errors
+
+`treeify` throws a `TypeError` when it can't render what it was given:
+
+- the input is not an array — `array-treeify: expected an array, received null`
+- the first element is not a string — `array-treeify: expected the first element to be a string, received number (1)`
+
+An empty array returns an empty string, so `treeify([])` is a safe way to say "nothing to render".
+
+### Types
+
+The exported `TreeInput` type (`Array<string | TreeInput>`) is intentionally permissive so trees can be assembled programmatically — build an array up with `push` and hand it over. A tuple type such as `[string, ...(string | TreeInput)[]]` *could* require a string first element at compile time, but it would rule out that dynamic construction, so the rule is enforced at runtime instead.
+
 ## Options
 
 - `chars`: Custom characters for the tree. Defaults to Unicode box-drawing characters.
diff --git a/src/index.test.ts b/src/index.test.ts
index 03e75f2..73552e2 100644
--- a/src/index.test.ts
+++ b/src/index.test.ts
@@ -183,15 +183,53 @@ second
 
   test('first element must be a string', () => {
     assert.throws(() => treeify([null, ['child']] as unknown as TreeInput), {
-      message: 'First element must be a string',
+      name: 'TypeError',
+      message:
+        'array-treeify: expected the first element to be a string, received null',
     })
     assert.throws(() => treeify([1, ['child']] as unknown as TreeInput), {
-      message: 'First element must be a string',
+      name: 'TypeError',
+      message:
+        'array-treeify: expected the first element to be a string, received number (1)',
     })
+    assert.throws(() => treeify([undefined] as unknown as TreeInput), {
+      name: 'TypeError',
+      message:
+        'array-treeify: expected the first element to be a string, received undefined',
+    })
+  })
+
+  test('input must be an array', () => {
+    for (const input of [null, undefined, 'root', 42, {}]) {
+      assert.throws(() => treeify(input as unknown as TreeInput), {
+        name: 'TypeError',
+        message: /^array-treeify: expected an array, received /,
+      })
+    }
   })
 
-  test('empty or invalid input returns empty string', () => {
+  test('empty input returns empty string', () => {
     assert.strictEqual(treeify([] as unknown as TreeInput), '')
-    assert.strictEqual(treeify([undefined] as unknown as TreeInput), '')
+  })
+
+  test('non-string labels are stringified rather than dropped', () => {
+    assert.strictEqual(
+      treeify(['root', [1, true, null]] as unknown as TreeInput),
+      `root
+├─ 1
+├─ true
+└─ null`,
+    )
+  })
+
+  test('a non-string label can be a parent node', () => {
+    assert.strictEqual(
+      treeify(['root', [2025, ['q1', 'q2'], 'note']] as unknown as TreeInput),
+      `root
+├─ 2025
+│  ├─ q1
+│  └─ q2
+└─ note`,
+    )
   })
 })
diff --git a/src/index.ts b/src/index.ts
index 1457d1a..58ae303 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -5,16 +5,36 @@
 export type TreeInput = Array<string | TreeInput>
 
 /**
- * Represents a node in the tree structure.
- * Can be either a string (a leaf node) or an array of TreeNodes (a branch with children).
+ * A value rendered as a node label. Anything that isn't an array is a label;
+ * non-strings are stringified.
  */
-type TreeNode = string | TreeNode[]
+type TreeLeaf = string | number | bigint | boolean
 
 /**
- * Flexible input type that accepts any array.
+ * Flexible input type that accepts labels and nested arrays.
  * Runtime validation ensures the first element is a string.
  */
-type FlexibleTreeInput = readonly (string | unknown[])[]
+type FlexibleTreeInput = readonly (TreeLeaf | readonly unknown[])[]
+
+/**
+ * @description An array is a branch (the children of the node before it);
+ * anything else is a leaf.
+ */
+function isBranch(node: unknown): node is readonly unknown[] {
+  return Array.isArray(node)
+}
+
+/**
+ * @description Describes a value for error messages without dumping its contents.
+ */
+function describe(value: unknown): string {
+  if (value === null) return 'null'
+  if (Array.isArray(value)) return 'an array'
+  const type = typeof value
+  if (type === 'number' || type === 'bigint' || type === 'boolean')
+    return `${type} (${String(value)})`
+  return type
+}
 
 /**
  * ASCII characters used to render the tree.
@@ -45,8 +65,9 @@ const EMPTY_CHARS: TreeChars = {
  *
  * The expected input format is a hierarchical structure where:
  * - The first element must be a string (the root node)
- * - String elements represent nodes at the current level
- * - Array elements following a string represent the children of the previous node
+ * - Label elements represent nodes at the current level. Anything that isn't an
+ *   array is a label, and non-strings are stringified
+ * - Array elements following a label represent the children of that node
  * - Nested arrays create deeper levels in the tree
  *
  * Examples of supported formats:
@@ -62,7 +83,9 @@ const EMPTY_CHARS: TreeChars = {
  *   - `chars` {TreeChars} - Custom characters for the tree. Defaults to Unicode box-drawing characters.
  *   - `plain` {boolean} - Whether to use plain whitespace characters instead of Unicode box-drawing characters.
  *
- * @returns {string} A string containing the tree representation
+ * @returns {string} A string containing the tree representation. An empty array returns an empty string.
+ *
+ * @throws {TypeError} If `list` is not an array, or its first element is not a string.
  *
  * @example
  * treeify(['root', ['child1', 'child2', ['grandchild']]])
@@ -78,10 +101,15 @@ export function treeify(
     plain?: boolean
   },
 ): string {
-  if (!Array.isArray(list) || list.length === 0) return ''
-  if (list[0] === undefined) return ''
+  if (!Array.isArray(list))
+    throw new TypeError(
+      `array-treeify: expected an array, received ${describe(list)}`,
+    )
+  if (list.length === 0) return ''
   if (typeof list[0] !== 'string')
-    throw new Error('First element must be a string')
+    throw new TypeError(
+      `array-treeify: expected the first element to be a string, received ${describe(list[0])}`,
+    )
 
   let chars = DEFAULT_CHARS
   if (options?.plain) chars = EMPTY_CHARS
@@ -95,18 +123,15 @@ export function treeify(
   while (i < list.length) {
     const node = list[i]
 
-    if (typeof node === 'string') {
-      // add strings here
-      result.push(node)
-      i++
-    } else if (Array.isArray(node)) {
+    if (isBranch(node)) {
       // array is the children of the previous item
       renderTreeNodes(node, '', result, chars)
-      i++
     } else {
-      // idk. skip it.
-      i++
+      // everything else is a label
+      result.push(String(node))
     }
+
+    i++
   }
 
   return result.join('\n')
@@ -116,51 +141,44 @@ export function treeify(
  * @description Renders tree nodes with appropriate ASCII indentation and branching
  */
 function renderTreeNodes(
-  nodes: TreeNode[],
+  nodes: readonly unknown[],
   indent: string,
   result: string[],
   chars: TreeChars,
 ): void {
-  if (!Array.isArray(nodes) || nodes.length === 0) return
+  if (nodes.length === 0) return
 
   const parentNodeIndices = findParentNodeIndices(nodes)
 
   let i = 0
   while (i < nodes.length) {
     const node = nodes[i]
-    const isParentNode = parentNodeIndices.has(i)
 
-    if (isParentNode) {
-      const parentIndex = i
-      const childrenIndex = i + 1
-      const stringNode = nodes[parentIndex] as string
-      const arrayNode = nodes[childrenIndex] as TreeNode[]
-
-      const isLast = !hasNextStringNode(nodes, childrenIndex + 1)
+    if (parentNodeIndices.has(i)) {
+      // a leaf followed by an array: that array holds its children
+      const children = nodes[i + 1] as readonly unknown[]
+      const isLast = !hasNextLeaf(nodes, i + 2)
 
       const prefix = isLast ? chars.lastBranch : chars.branch
-      result.push(indent + prefix + stringNode)
+      result.push(indent + prefix + String(node))
 
       // children with increased indent
       const childIndent = indent + (isLast ? chars.space : chars.pipe)
-      renderTreeNodes(arrayNode, childIndent, result, chars)
+      renderTreeNodes(children, childIndent, result, chars)
 
       // skip both the parent node and its children array
       i += 2
-    } else if (typeof node === 'string') {
-      // string is simple. add it.
-      const isLast = !hasNextStringNode(nodes, i + 1)
-      const prefix = isLast ? chars.lastBranch : chars.branch
-      result.push(indent + prefix + node)
-      i++
-    } else if (Array.isArray(node)) {
-      // (>_>)
+    } else if (isBranch(node)) {
+      // an array with no leaf before it. (>_>) render it a level deeper.
       const isLast = i === nodes.length - 1
       const childIndent = indent + (isLast ? chars.space : chars.pipe)
       renderTreeNodes(node, childIndent, result, chars)
       i++
     } else {
-      // (0_o)
+      // a leaf is simple. add it.
+      const isLast = !hasNextLeaf(nodes, i + 1)
+      const prefix = isLast ? chars.lastBranch : chars.branch
+      result.push(indent + prefix + String(node))
       i++
     }
   }
@@ -169,15 +187,11 @@ function renderTreeNodes(
 /**
  * @description Locate parent nodes in the array to handle nesting.
  */
-function findParentNodeIndices(nodes: TreeNode[]): Set<number> {
+function findParentNodeIndices(nodes: readonly unknown[]): Set<number> {
   const parentNodeIndices = new Set<number>()
 
   for (let i = 0; i < nodes.length; i++) {
-    if (
-      typeof nodes[i] === 'string' &&
-      i + 1 < nodes.length &&
-      Array.isArray(nodes[i + 1])
-    ) {
+    if (!isBranch(nodes[i]) && isBranch(nodes[i + 1])) {
       parentNodeIndices.add(i)
     }
   }
@@ -187,12 +201,12 @@ function findParentNodeIndices(nodes: TreeNode[]): Set<number> {
 
 /**
  * @description
- *   Determines if there's another string node after the given index.
+ *   Determines if there's another leaf after the given index.
  *   Used to decide if the current node is the last at its level.
  */
-function hasNextStringNode(nodes: TreeNode[], startIndex: number): boolean {
+function hasNextLeaf(nodes: readonly unknown[], startIndex: number): boolean {
   for (let i = startIndex; i < nodes.length; i++) {
-    if (typeof nodes[i] === 'string') {
+    if (!isBranch(nodes[i])) {
       return true
     }
   }
diff --git a/src/readme-examples.test.ts b/src/readme-examples.test.ts
index 9655b9b..493f840 100644
--- a/src/readme-examples.test.ts
+++ b/src/readme-examples.test.ts
@@ -115,4 +115,19 @@ describe('readme examples', () => {
     console.log(result)
     assert.strictEqual(result, expected)
   })
+
+  test('labels example', () => {
+    const deploys = ['deploys', [2025, ['spring', 'summer'], 2026, ['q1']]]
+    const expected = `deploys
+├─ 2025
+│  ├─ spring
+│  └─ summer
+└─ 2026
+   └─ q1`
+
+    const result = treeify(deploys)
+    console.log('\nLabels example:')
+    console.log(result)
+    assert.strictEqual(result, expected)
+  })
 })
diff --git a/src/types.test.ts b/src/types.test.ts
index 9592570..92a4358 100644
--- a/src/types.test.ts
+++ b/src/types.test.ts
@@ -23,7 +23,9 @@ describe('treeify types', () => {
     // @ts-expect-error
     const inputWithoutRootString: TreeInput = [{ bad: 'root' }, 'root2']
     assert.throws(() => treeify(inputWithoutRootString), {
-      message: 'First element must be a string',
+      name: 'TypeError',
+      message:
+        'array-treeify: expected the first element to be a string, received object',
     })
 
     const inputWithRootStringAndInvalidValues: TreeInput = ['root']
@@ -32,7 +34,16 @@ describe('treeify types', () => {
     // @ts-expect-error
     inputWithRootStringAndInvalidValues.push({}, [], Number.POSITIVE_INFINITY)
     result = treeify(inputWithRootStringAndInvalidValues)
-    assert.ok(result) // non-strings are ignored
+    // non-strings are stringified, empty arrays render nothing
+    assert.equal(result, 'root\n1\n[object Object]\nInfinity')
+  })
+
+  test('non-string labels need no cast', () => {
+    const result = treeify(['deploys', [2025, ['spring', 'summer'], true]])
+    assert.equal(
+      result,
+      'deploys\n├─ 2025\n│  ├─ spring\n│  └─ summer\n└─ true',
+    )
   })
 
   test('generated inputs', () => {