diff --git a/test/optional-parameter.test.js b/test/optional-parameter.test.js
new file mode 100644
index 00000000..a9f6fb7a
--- /dev/null
+++ b/test/optional-parameter.test.js
@@ -0,0 +1,20 @@
+// @ts-check
+import { expect, test } from 'vitest';
+import { acornParse } from './common.js';
+import { print } from '../src/index.js';
+import ts from '../src/languages/ts/index.js';
+
+/** @param {string} input */
+function printed(input) {
+	const { ast } = acornParse(input);
+	return print(ast, ts()).code;
+}
+
+// `?` on optional identifiers must be preserved (see #139)
+test.each([
+	'const f = (o?: { A?: string }) => {};',
+	'function a(disabled?) {}',
+	'function openEventForm(startDate?, endDate?, allDay = false) {}'
+])('preserves `?` on optional identifiers: %j', (input) => {
+	expect(printed(input)).toBe(input);
+});