diff --git a/src/Monolog/Utils.php b/src/Monolog/Utils.php
index 007ad686..7515eadb 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, $length ?: strlen($string));
     }
 
     /**
diff --git a/test.php b/test.php
new file mode 100644
index 00000000..e69de29b