diff --git a/src/Monolog/Utils.php b/src/Monolog/Utils.php
index 007ad686..5299d441 100644
--- a/src/Monolog/Utils.php
+++ b/src/Monolog/Utils.php
@@ -31,7 +31,7 @@ final class Utils
             return mb_strcut($string, $start, $length);
         }
 
-        return substr($string, $start, $length);
+        return substr($string, $start, (null === $length) ? strlen($string) : $length);
     }
 
     /**