Error in Add only custom permission

I have created library in that i have folders and subfolders my aim is to restrict users to add only items in the subfolders not to add or edit or rename folders and subfolders just need to give permission to add items so i created custom permission same as contribute and uncheck edit items and delete items but when i apply this permission to folders its not allowing me to create items inside the folder can anyone help me need to do any changes ?? thanks