diff --git a/tests/Formatter/ATFormatterTest.php b/tests/Formatter/ATFormatterTest.php
index fca5613..d75d562 100644
--- a/tests/Formatter/ATFormatterTest.php
+++ b/tests/Formatter/ATFormatterTest.php
@@ -32,6 +32,7 @@ public function providerFormat() : array
             ['1', null],
             ['12', null],
             ['123', null],
+            ['0123', null],
             ['1234', '1234'],
             ['12345', null],
             ['123456', null],