Recursive call return a List, return type causing me issues

I have a recursive method that returns categories, and checks for its sub categories.

This is my code:

public List<Category> GetAllChildCats(int categoryid) 
{
    List<Category> list = new List<Category>();

    Category c = Get(categoryid);

    foreach(Category cat in c.ChildCategories) 
    {
        list.Add(GetAllChildCats(cat.CategoryID));
    }

}

This fails because the call to list.Add() expects a Category object, but GetAllChildCats() returns List<Category>

How should I work around this?