Index: src/wp-includes/user.php
===================================================================
--- src/wp-includes/user.php	(revision 38768)
+++ src/wp-includes/user.php	(working copy)
@@ -2440,7 +2440,7 @@
 	}
 
 	$prefix = $wpdb->get_blog_prefix();
-	$regex  = implode( '|', wp_roles()->get_names() );
+	$regex  = implode( '|', array_keys( wp_roles()->get_names() ) );
 	$regex  = preg_replace( '/[^a-zA-Z_\|-]/', '', $regex );
 	$users  = $wpdb->get_col( $wpdb->prepare( "
 		SELECT user_id