python递归oop vs功能

我很好奇为什么第一个函数确实起作用（OOP），但是第二个触发器却出错了。有人可以解释吗？

class Solution(object):
    def __init__(self):
        self.solved = {0:0,1:1,2:1}

    def fibonacci(self, n):
        """
        :type n: int
        :rtype: int
        """
        if n in self.solved.keys():
            return self.solved[n]
        else:
            value = self.fibonacci(n-1) + self.fibonacci(n-2)

        self.solved[n] = value
        return value

t = Solution()
t.fibonacci(7)
>>> 13

但这行不通

found = {0:0,1:1,2:1}
def fibonacci(n):
    if n in found.keys():
        return found[n]
    else:
        value = fibonacci(n-1) + fibonacci(n-2)

    found[n] = value
    return value
fibonacci(7)
>>>TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'