It uses null now. Output is specified for tested inputs. Shall we leave it at that?

Added some random cases for ruby

I would agree with you, but I think the purpose of the grasshopper series is to be completable by middle school students.

The test is on 9876543210. You are returning 8976543210 instead of -1.
You need to handle the case of numbers for which no higher number exists.

Your function has to return true or false depending on whether the coordinates are valid or not.
match doesn't do that. It returns null or an array containing the matches.

While the first 13 tests use Test.expect which works with falsy and truthy values, the random tests actually check if you return true/false instead of some array or null.

I can't really say what the 14th test is since it's randomly decided (after 13 fixed tests).
But considering there have been more than 800 correct submissions, I have reason to believe the problem lies in your code.

Could you post your code here?

or I'd rather return undefined in this case, not null