Python代码运行后报错maximum recursion depth exceeded in c

代码如下:

def demo(num):
    if num<1:
        return
    demo(num)
    return num+1



print(demo(2))

运行报错

Traceback (most recent call last):
  File "/Users/jiangyd/PycharmProjects/untitled16/demo2/b.py", line 10, in <module>
    print(demo(2))
  File "/Users/jiangyd/PycharmProjects/untitled16/demo2/b.py", line 5, in demo
    demo(num)
  File "/Users/jiangyd/PycharmProjects/untitled16/demo2/b.py", line 5, in demo
    demo(num)
  File "/Users/jiangyd/PycharmProjects/untitled16/demo2/b.py", line 5, in demo
    demo(num)
  [Previous line repeated 995 more times]
  File "/Users/jiangyd/PycharmProjects/untitled16/demo2/b.py", line 3, in demo
    if num<1:
RecursionError: maximum recursion depth exceeded in comparison