We can't tell based on that little fragment.
If I use that in an SQL statement and run it against my DB:
SELECT TOP 10
CASE WHEN Id IS NULL
THEN 0
ELSE 1
END AS IsParent
FROM MyTable
It works fine: I get a single column containing 10 1's or 0's.
So you need to look at your SP in more detail, and work out where that code fragment is being used and what the rest of the SP / your app is doing with that result - somewhere it's expecting a boolean and getting that integer, but we have no idea where!
We can't do that for you: we don't have access to anything except exactly what you type!