also I feel like an option type would be much more idomatic

but this is perhaps not exactly a Kotlin-specific issue

I think you need to read this: https://kotlinlang.org/docs/reference/java-interop.html#null-safety-and-platform-types

So it happily returns

null

And the interface can't check whether the non-nullability is enforced because it's an interface, and the calling code trusts the interface, checking for

null

on every single call would be infeasible.

I'd say the solution is to only use nullable types for functions like this.

It's actually the Dao codegen that should see the

@NotNull

annotation on the interface function and refuse to compile.