I tried your code; I get:

Fixed_Tests
Caught std::exception, what(): vector::_M_range_check: __n (which is 1) >= this->size() (which is 0)

Note there are more fixed tests when you "Attempt".

Is there some sort of mistake in the tests here

No. Look at the top of the page: 2719 people passed the C++ kata. Before posting issues look how many people passed the kata in your language. If there were errors somebody would have seen them before you... and you would avoid to make people waste their time:-)
Reading this: "https://docs.codewars.com/training/troubleshooting could help you.
Good luck!

Your code seems to pass now. Did you find your mistake?

What language?