diff --git a/src/wp-admin/includes/user.php b/src/wp-admin/includes/user.php
index bb68ad9c62..5dbdafd629 100644
--- a/src/wp-admin/includes/user.php
+++ b/src/wp-admin/includes/user.php
@@ -143,7 +143,7 @@ function edit_user( $user_id = 0 ) {
 		$errors->add( 'user_login', __( '<strong>ERROR</strong>: This username is already registered. Please choose another one.' ));
 
 	/** This filter is documented in wp-includes/user-functions.php */
-	$illegal_logins = apply_filters( 'illegal_user_logins', array() );
+	$illegal_logins = (array) apply_filters( 'illegal_user_logins', array() );
 
 	if ( in_array( strtolower( $user->user_login ), array_map( 'strtolower', $illegal_logins ) ) ) {
 		$errors->add( 'illegal_user_login', __( '<strong>ERROR</strong>: Sorry, that username is not allowed.' ) );
diff --git a/src/wp-includes/ms-functions.php b/src/wp-includes/ms-functions.php
index d54a838e10..ac1d5c6d80 100644
--- a/src/wp-includes/ms-functions.php
+++ b/src/wp-includes/ms-functions.php
@@ -432,7 +432,7 @@ function wpmu_validate_user_signup($user_name, $user_email) {
 	}
 
 	/** This filter is documented in wp-includes/user-functions.php */
-	$illegal_logins = apply_filters( 'illegal_user_logins', array() );
+	$illegal_logins = (array) apply_filters( 'illegal_user_logins', array() );
 
 	if ( in_array( strtolower( $user_name ), array_map( 'strtolower', $illegal_logins ) ) ) {
 		$errors->add( 'user_name',  __( 'Sorry, that username is not allowed.' ) );
diff --git a/src/wp-includes/user-functions.php b/src/wp-includes/user-functions.php
index 1a71fcaaad..2b8151fd3d 100644
--- a/src/wp-includes/user-functions.php
+++ b/src/wp-includes/user-functions.php
@@ -1328,7 +1328,7 @@ function wp_insert_user( $userdata ) {
 	 *
 	 * @param array $usernames Array of blacklisted usernames.
 	 */
-	$illegal_logins = apply_filters( 'illegal_user_logins', array() );
+	$illegal_logins = (array) apply_filters( 'illegal_user_logins', array() );
 
 	if ( in_array( strtolower( $user_login ), array_map( 'strtolower', $illegal_logins ) ) ) {
 		return new WP_Error( 'illegal_user_login', __( 'Sorry, that username is not allowed.' ) );