Explain yourself plz. I tried putting "testing" and it says:

Traceback (most recent call last):
  File "tests.py", line 3, in <module>
    from solution import super_ints as si
ImportError: cannot import name 'super_ints' from 'solution' (/workspace/default/solution.py)

I don't get what you mean by "Test cases can pass if you put a string".
Can you elabrate your issue?

at first the solution was placed in preloaded, so any string in solution area counted as correct solution. But now author fixed this problem.

Explain yourself, plz. What's the problem?