Cannot include Kotlin "ad platform type" warning in error message

Question

I have a simple Kotlin function that calls some Java code:

fun simpleSchema(field: GraphQLFieldDefinition) =
    newSchema()
        .query(newObject()
            .name("root")
            .field(field)
            .build())
        .build()

Inside my IDEA, namely

IntelliJ IDEA 2016.2.5
Build #IU-162.2228.15, built on October 14, 2016
JRE: 1.8.0_112-release-287-b2 amd64
JVM: OpenJDK 64-Bit Server VM by JetBrains s.r.o

there is a useful message:

Declaration has platform type. Make the type explicit to prevent subtle bugs.
This inspection reports functions and properties that have platform type. 
In order to prevent unexpected errors, the type should be declared explicitly.

which is great. But on the RHS editor panel, this message is painted as a weak warning, and I want this to be a mistake. I go to Settings → Editor → Inspections → Kotlin → "A function or property has a platform type" and is flagged as an error.

Why is this not working? A search for “platform type” in checks shows a more likely candidate than higher. I restarted and canceled caching ...