UCHAR_MAX? You're overwriting something you shouldn't.

It looks like the problem might be in the if-statement, the else-clause in particular:

else {
      *(dst++) = *(src++);
  }

The kata spec requests that vowels' cases be swapped, however the nature of the if-statement in the code does not seem to address the uppercase vowels. Uppercase vowels follow the else-clause, in which case they are only copied, but don't have their case swapped.

Thanks for the report, null_ptr! I'll look into the issue ASAP.

Done... If you have any troubles write again. Now the random tests are without leading and trailing zeroes.