@Voila your answer doesn't help with anything

There are both tests with Int and Integer. Declare the function type so that it can work with either or just don't declare the type.

I experienced this as well

I don't think that's the case. When I changed the type decleration to [Int] -> String I still got an error:

/tmp/haskell11799-5-qnsh3p.q8iz1if6r/Test/OddOrEven.hs:30:21:
Couldn't match type Integer' with Int'
Expected type: [Int]
Actual type: [Integer]
In the first argument of oddOrEven', namely a'
In the first argument of shouldBe', namely oddOrEven a'

I think there's a deeper problem with the tests for this kata.

Read the error carefully:

Couldn't match type Int with Integer
Expected type: [Integer]
Actual type: [Int]

Your function's type signature is [Integer], but it should actually be [Int].