diff --git a/tests/phpunit/tests/taxonomy.php b/tests/phpunit/tests/taxonomy.php
index 7d5bff4588..6d9cc117cc 100644
--- a/tests/phpunit/tests/taxonomy.php
+++ b/tests/phpunit/tests/taxonomy.php
@@ -127,6 +127,8 @@ class Tests_Taxonomy extends WP_UnitTestCase {
 
 	/**
 	 * @ticket 21593
+	 *
+	 * @expectedIncorrectUsage register_taxonomy
 	 */
 	function test_register_long_taxonomy() {
 		$this->assertInstanceOf( 'WP_Error', register_taxonomy( 'abcdefghijklmnopqrstuvwxyz0123456789', 'post', array() ) );