I need to write in Kotlin DSL: filter {line -> if (true/*some condition*/) line else null } Filter is defined here: https://docs.gradle.org/current/dsl/org.gradle.api.tasks.Copy.html#org.gradle.api.tasks.Copy:filter(org.gradle.api.Transformer) But compiler does not compile it, because null is not String. Filter is defined in java and null is allowed there. How to write it in Kotlin?

Is the Java type declared as non null? Else that should work

They are not declared as not null: public AbstractCopyTask filter(Transformer<String, String> transformer) {....