Input validation does not add anything of value.

Those cases are already excluded with the assumption in the description:

You can assume the given code is syntactically valid, i.e. it only consists of numbers and it exactly has a length of 13 characters.

Also, even if someone would try adding those test cases, it would invalidate the majority of the current ~2800 solutions, so I (not the author but a mender) don't think it would be worth it, honestly...

You can add test case:

• when length of line less 13
• when length of line greate 13
• when line not only digit

Thank's for your task