I personally find this really strange. I receive the same error in Ruby. While I appreciate that the final answer should be an array, I don't expect to see a Ruby error if I return an integer - just a failing test, telling me what actually was returned and what was expected.

So, is the example test case right or wrong? As I too count 4. See:

[5, 7]
[5, 9]
[6, 8]
[7, 9]

100%. 12, in this form, is not a word. I also find it odd that out of 100 test cases, only one contains a number.