Throw an exception on invalid level name

src/Monolog/Logger.php

@@ -322,6 +322,10 @@ class Logger
      */
     public static function getLevelName($level)
     {
+        if (!isset(static::$levels[$level])) {
+            throw new \InvalidArgumentException('Level "'.$level.'" is not defined, use one of: '.implode(', ', array_keys(static::$levels)));
+        }
+
         return static::$levels[$level];
     }