Kotlin resource endpoint doesn't return JSON

Question

I have a small Kotling resource, where I want to return a JSON representation of a class, but it seems to return some strange format:

CatFact(fact=This is a cat, length=2)

This resource is run in a Quarkus application with RESTEasy. Could that be the issue? I was assuming that this resource should map the object to JSON automatically:

package org.polychat.resources

import com.fasterxml.jackson.annotation.JsonProperty
import jakarta.ws.rs.GET
import jakarta.ws.rs.Path
import jakarta.ws.rs.Produces
import jakarta.ws.rs.core.MediaType

data class CatFact(
        @JsonProperty("fact") val fact: String,
        @JsonProperty("length") val length: Int,
)

@Path("/kotlin")
class KotlinResource {

    @GET
    @Produces(MediaType.TEXT_PLAIN)
    fun helloKotlin() = "Hello from Kotlin RESTEasy Reactive"

    @GET
    @Produces(MediaType.APPLICATION_JSON)
    @Path("/catfact")
    fun catFact(): CatFact? {
        val catFact = CatFact("This is a cat", 2)
        return catFact
    }
}