The error message is reasonable because QuickCheck is telling you all about what it can tell you. The problem is:

1. You need to guess what it means not because the error message itself is incomprehensible, but because there's no context: The test isn't shown
2. The reference solution is messed up

Yeah, incomprehensible error message:

Falsifiable (after 3 tests and 2 shrinks): 
expected: Just 0
 but got: Nothing
[0]
Positive {getPositive = 1}
-1