Kotlin grass missing field parsing error

example.kt

import com.github.doyaaaaaken.kotlincsv.dsl.csvReader
import io.blackmo18.kotlin.grass.dsl.grass

val csv = """
firstName,lastName,email
John,Doe,[email protected]    
""".trimIndent()

data class Contact(
    val firstName: String?,
    val lastName: String?,
    val email: String?,
    val location: String?,
)

@OptIn(ExperimentalStdlibApi::class)
fun parseContacts(csv: String) =
csvReader().readAllWithHeader(csv).let { 
    grass<Contact> {
        ignoreUnknownFields = true
    }.harvest(it)
}

val contacts = parseContacts(csv)

println(contacts)

stacktrace

java.lang.IllegalArgumentException: argument type mismatch
	at java.base/jdk.internal.reflect.DirectConstructorHandleAccessor.newInstance(DirectConstructorHandleAccessor.java:70)
	at java.base/java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:500)
	at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:484)
	at kotlin.reflect.jvm.internal.calls.CallerImpl$Constructor.call(CallerImpl.kt:41)
	at kotlin.reflect.jvm.internal.KCallableImpl.call(KCallableImpl.kt:108)
	at io.blackmo18.kotlin.grass.pot.Stem.harvestData(Stem.kt:49)
	at io.blackmo18.kotlin.grass.pot.Plant.harvest(Plant.kt:42)
	at Line_4.parseContacts(Line_4.kts:6)
	at Line_5.<init>(Line_5.kts:1)
Caused by: java.lang.ClassCastException: Cannot cast kotlin.Unit to java.lang.String
	at java.base/java.lang.Class.cast(Class.java:3946)
	at java.base/jdk.internal.reflect.DirectConstructorHandleAccessor.newInstance(DirectConstructorHandleAccessor.java:67)