diff --git a/src/wp-admin/includes/user.php b/src/wp-admin/includes/user.php
index 20fbcd851f..8f40723e67 100644
--- a/src/wp-admin/includes/user.php
+++ b/src/wp-admin/includes/user.php
@@ -193,7 +193,7 @@ function edit_user( $user_id = 0 ) {
 	/** This filter is documented in wp-includes/user.php */
 	$illegal_logins = (array) apply_filters( 'illegal_user_logins', array() );
 
-	if ( in_array( strtolower( $user->user_login ), array_map( 'strtolower', $illegal_logins ) ) ) {
+	if ( in_array( strtolower( $user->user_login ), array_map( 'strtolower', $illegal_logins ), true ) ) {
 		$errors->add( 'invalid_username', __( '<strong>ERROR</strong>: Sorry, that username is not allowed.' ) );
 	}
 
diff --git a/src/wp-includes/ms-functions.php b/src/wp-includes/ms-functions.php
index 94418ad350..43325c0cac 100644
--- a/src/wp-includes/ms-functions.php
+++ b/src/wp-includes/ms-functions.php
@@ -488,7 +488,7 @@ function wpmu_validate_user_signup( $user_name, $user_email ) {
 	/** This filter is documented in wp-includes/user.php */
 	$illegal_logins = (array) apply_filters( 'illegal_user_logins', array() );
 
-	if ( in_array( strtolower( $user_name ), array_map( 'strtolower', $illegal_logins ) ) ) {
+	if ( in_array( strtolower( $user_name ), array_map( 'strtolower', $illegal_logins ), true ) ) {
 		$errors->add( 'user_name', __( 'Sorry, that username is not allowed.' ) );
 	}
 
diff --git a/src/wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php b/src/wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php
index dacc327b0e..652f4ead76 100644
--- a/src/wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php
+++ b/src/wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php
@@ -1137,7 +1137,7 @@ class WP_REST_Users_Controller extends WP_REST_Controller {
 		/** This filter is documented in wp-includes/user.php */
 		$illegal_logins = (array) apply_filters( 'illegal_user_logins', array() );
 
-		if ( in_array( strtolower( $username ), array_map( 'strtolower', $illegal_logins ) ) ) {
+		if ( in_array( strtolower( $username ), array_map( 'strtolower', $illegal_logins ), true ) ) {
 			return new WP_Error( 'rest_user_invalid_username', __( 'Sorry, that username is not allowed.' ), array( 'status' => 400 ) );
 		}
 
diff --git a/src/wp-includes/user.php b/src/wp-includes/user.php
index f6b60db913..fd03e6994e 100644
--- a/src/wp-includes/user.php
+++ b/src/wp-includes/user.php
@@ -1584,7 +1584,7 @@ function wp_insert_user( $userdata ) {
 	 */
 	$illegal_logins = (array) apply_filters( 'illegal_user_logins', array() );
 
-	if ( in_array( strtolower( $user_login ), array_map( 'strtolower', $illegal_logins ) ) ) {
+	if ( in_array( strtolower( $user_login ), array_map( 'strtolower', $illegal_logins ), true ) ) {
 		return new WP_Error( 'invalid_username', __( 'Sorry, that username is not allowed.' ) );
 	}
 
@@ -2503,7 +2503,7 @@ function register_new_user( $user_login, $user_email ) {
 	} else {
 		/** This filter is documented in wp-includes/user.php */
 		$illegal_user_logins = (array) apply_filters( 'illegal_user_logins', array() );
-		if ( in_array( strtolower( $sanitized_user_login ), array_map( 'strtolower', $illegal_user_logins ) ) ) {
+		if ( in_array( strtolower( $sanitized_user_login ), array_map( 'strtolower', $illegal_user_logins ), true ) ) {
 			$errors->add( 'invalid_username', __( '<strong>ERROR</strong>: Sorry, that username is not allowed.' ) );
 		}
 	}