Roman Numerals check issue

I would like to give some feedback about the pass test for the Roman Numerals problem. When I go to check my results, the checkio environment finds that:

checkio(1) returns null, when it should return 'I'

But when I run my code in the IDLE environment on my computer, it produces a 'I' when given checkio(1).

(my code is attached)

From: https://py.checkio.org/mission/roman-numerals/solve/

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