PyCharm EDU | Checker's error: infinite loop while revising student's code

Lesson 7 task 3:

• correct solution that doesn't cause endless loop

def fib(n):
    result = []
    a = 1
    b = 1               # first editable field
    while a < n:
        result.append(a)
        tmp_var = b
        b = a + b       # second editable field
        a = tmp_var     # third editable field
    return result

print(fib(10))

# output: [1, 1, 2, 3, 5, 8]

• another correct solution that cases endless loop and RAM overloading

def fib(n):
    result = []
    a = 1
    b = 0               # first editable field
    while a < n:
        result.append(a)
        tmp_var = b
        b = a            # second editable field
        a += tmp_var     # third editable field
    return result

print(fib(10))

# output: [1, 1, 2, 3, 5, 8]

What is wrong with it?

I am intensely interested to know! Do you have any idea?