Hi @evgeniyakarna and welcome to Codewars!

I tested your solution and noticed that the errors seem to occur when letter 'Z' is in original string.

Because your code takes 'Z' and turns it to 'a', this is the error I think - remember, "Makes any vowels capital" so it should take 'Z' to 'A'.

For a useful general piece of advice for your Codewars journey, on this Troubleshooting page it recommends to "Print the input" when you have some problems with your code. In Python this means:

def changer(s):
    print(s) # <--- ADD THIS LINE
    # rest of your code as usual...
    next_let_word = str()
    ...

Now, in Codewars console you can see which input (here, s) causes your code to fail - that's how I was able to debug your solution. This is useful technique that will help in 99% of katas if you need to debug.