ANTLR plugin with Kotlin

I’m moving over an existing Gradle project to Kotlin and I’ve run into an issue where I can’t seem to override the source dir for the ANTLR plugin. The plugin adds an antlr property to the source sets but I cannot find any way to set it via Kotlin:

build.gradle (Groovy) example:

sourceSets {
    customSourceSet {
        antlr.srcDirs = ["$projectDir/customPath"]

build.gradle.kts (Kotlin) non-working, ‘antlr’ is unresolved:

sourceSets["customSourceSet"].antlr {

Following up to see if anyone has any idea on this. Thanks!