paddle.fluild常量计算报错‘NoneType‘ object has no attribute ‘get_fetch_list‘

错误来源：

《paddlepaddle Fluid深度学习入门与实》书籍中第一个示例：“PaddlePaddle常量的使用”

示例代码如下：

import paddle.fluid as fluid

x1 = fluid.layers.fill_constant(shape=[2, 2], value=1, dtype='int64')
x2 = fluid.layers.fill_constant(shape=[2, 2], value=1, dtype='int64')

y1 = fluid.layers.sum(x=[x1, x2])

place = fluid.CPUPlace()
exe = fluid.executor.Executor(place)

exe.run(fluid.default_startup_program())
result = exe.run(program=fluid.default_main_program(), fetch_list=[y1])
print(result)

if __name__ == '__main__':
    pass

运行报错如下：

C:\ProgramData\Anaconda3\lib\site-packages\paddle\fluid\executor.py:1279: UserWarning: There are no operators in the program to be executed. If you pass Program manually, please use fluid.program_guard to ensure the current Program is being used.
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