GO tests have the same issue, I think. I have 3 test cases that certainly has a next smaller but wuold not have a next larger (in the case of next larger the -1 would make sense).

Basic Tests
should test that the solution returns the correct value
Expected
: 721
to equal
: -1
Completed in 0.4325ms
Long Tests
should test that the solution returns the correct value
Expected
: 987654321
to equal
: -1
Completed in 0.3350ms
Sample Tests
should test that the solution returns the correct value
Extended Tests
should test that the solution returns the correct value
Expected
: 10
to equal
: -1

It should be noted that if I keep re-attempting until I get a randomization that does not cross from positive ints to negative ints, my solution passes.

The randomized test cases produce cases that fail incorrectly. When I have a range that crosses the negative to positive boundary (i.e. "-1, 0, 1, 2") the test case insistes that the output be "-1, 0-2" when according to the requirements it should be "-1-2".

The desription specifically says that there will be no punctuation. I thought the same as you but when I reread the description I found that punctuation was not something I had to worry about :)