MapStruct Property has no write accessor in MetaData for target name - kotlin

Question

I have a FlatDTO that needs to be mapped to a nested Response containing InfoData and MetaData
The code for the response is generated by OpenAPI. So the below definitions can't be changed.
package org.mapstruct.example.kotlin.openapi
import com.fasterxml.jackson.annotation.JsonProperty
import javax.validation.Valid
data class Response(
#field:Valid #field:JsonProperty("infoData", required = true) val infoData: InfoData,
#field:Valid #field:JsonProperty("metaData", required = true) val metaData: MetaData
)
data class InfoData(
#field:JsonProperty("id", required = true) val id: kotlin.String,
)
data class MetaData(
#field:JsonProperty("firstProperty") val firstProperty: String? = null,
)
I have defined FlatDTO as follows.
package org.mapstruct.example.kotlin.models
data class FlatDTO(
var id: String? = null,
var firstProperty: String,
)
Here is my mapper class which maps FlatDTO to Response
package org.mapstruct.example.kotlin.mapper
import org.mapstruct.Mapper
import org.mapstruct.Mapping
import org.mapstruct.Mappings
import org.mapstruct.example.kotlin.models.FlatDTO
import org.mapstruct.example.kotlin.openapi.Response
#Mapper
interface DataMapper {
#Mappings(
Mapping(target = "infoData.id", source = "id"),
Mapping(target = "metaData.firstProperty", source = "firstProperty")
)
fun flatToResponse(flatDTO: FlatDTO): Response
}
When I try to build the code using mvn clean install
I get the following error.
error: Property "firstProperty" has no write accessor in MetaData for target name "metaData.firstProperty".
[ERROR] #org.mapstruct.Mappings(value = {#org.mapstruct.Mapping(target = "infoData.id", source = "id"), #org.mapstruct.Mapping(target = "metaData.firstProperty", source = "firstProperty")})
I understand that this message is trying to say that there is no setter function for firstProperty because it's defined as val but that code cannot be edited. I can write my own custom mapper that works just fine.
I wanted to understand if there is a way to use MapStruct in this scenario.