diff --git a/packages/i18n/src/index.js b/packages/i18n/src/index.js
index dc179517312014..639325479b7101 100644
--- a/packages/i18n/src/index.js
+++ b/packages/i18n/src/index.js
@@ -39,8 +39,8 @@ const i18n = new Tannin( {} );
  *
  * @see http://messageformat.github.io/Jed/
  *
- * @param {?Object} data   Locale data configuration.
- * @param {?string} domain Domain for which configuration applies.
+ * @param {Object} [data]   Locale data configuration.
+ * @param {string} [domain] Domain for which configuration applies.
  */
 export function setLocaleData( data, domain = 'default' ) {
 	i18n.data[ domain ] = {
@@ -61,12 +61,12 @@ export function setLocaleData( data, domain = 'default' ) {
  * Wrapper for Tannin's `dcnpgettext`. Populates default locale data if not
  * otherwise previously assigned.
  *
- * @param {?string} domain  Domain to retrieve the translated text.
- * @param {?string} context Context information for the translators.
+ * @param {string|undefined} domain  Domain to retrieve the translated text.
+ * @param {string|undefined} context Context information for the translators.
  * @param {string}  single  Text to translate if non-plural. Used as fallback
  *                          return value on a caught error.
- * @param {?string} plural  The text to be used if the number is plural.
- * @param {?number} number  The number to compare against to use either the
+ * @param {string} [plural]  The text to be used if the number is plural.
+ * @param {number} [number]  The number to compare against to use either the
  *                          singular or plural form.
  *
  * @return {string} The translated string.
@@ -85,7 +85,7 @@ function dcnpgettext( domain = 'default', context, single, plural, number ) {
  * @see https://developer.wordpress.org/reference/functions/__/
  *
  * @param {string}  text   Text to translate.
- * @param {?string} domain Domain to retrieve the translated text.
+ * @param {string} [domain] Domain to retrieve the translated text.
  *
  * @return {string} Translated text.
  */
@@ -100,7 +100,7 @@ export function __( text, domain ) {
  *
  * @param {string}  text    Text to translate.
  * @param {string}  context Context information for the translators.
- * @param {?string} domain  Domain to retrieve the translated text.
+ * @param {string} [domain]  Domain to retrieve the translated text.
  *
  * @return {string} Translated context string without pipe.
  */
@@ -114,11 +114,11 @@ export function _x( text, context, domain ) {
  *
  * @see https://developer.wordpress.org/reference/functions/_n/
  *
- * @param {string}  single The text to be used if the number is singular.
- * @param {string}  plural The text to be used if the number is plural.
- * @param {number}  number The number to compare against to use either the
- *                         singular or plural form.
- * @param {?string} domain Domain to retrieve the translated text.
+ * @param {string}  single  The text to be used if the number is singular.
+ * @param {string}  plural  The text to be used if the number is plural.
+ * @param {number}  number  The number to compare against to use either the
+ *                          singular or plural form.
+ * @param {string} [domain] Domain to retrieve the translated text.
  *
  * @return {string} The translated singular or plural form.
  */
@@ -137,7 +137,7 @@ export function _n( single, plural, number, domain ) {
  * @param {number}  number  The number to compare against to use either the
  *                          singular or plural form.
  * @param {string}  context Context information for the translators.
- * @param {?string} domain  Domain to retrieve the translated text.
+ * @param {string} [domain] Domain to retrieve the translated text.
  *
  * @return {string} The translated singular or plural form.
  */