diff --git a/doc/source/tutorial/customizeRL.md b/doc/source/tutorial/customizeRL.md
index ed96459a0..ac0ddedc4 100644
--- a/doc/source/tutorial/customizeRL.md
+++ b/doc/source/tutorial/customizeRL.md
@@ -14,7 +14,7 @@ from gym.spaces import Box
 
 
 class MyEnv(gym.Env):
-    def __init__(cfg, **kwargs):
+    def __init__(self, cfg, **kwargs):
         ...
         obs_shape = ...
         act_shape = ...