RandomClassProvider has special handling for types like primitives and enums that don't have constructors. However, this special handling is only applied when mapping over direct constructor parameters. This means that constructor-less classes don't get built correctly if they are built under other circumstances, such as when they are included in a collection. For example:

package com.mr.reproducer

import io.github.serpro69.kfaker.Faker
import org.junit.jupiter.api.Test

data class Works(val case : AnEnum)

data class DoesNotWork(val case : List<AnEnum>)

enum class AnEnum {
    FOO,
    BAR
}

class TestCases {

    @Test
    fun `special handling occurs`() {
        Faker().randomProvider.randomClassInstance<Works>()
    }

    @Test
    fun `special handling does not occur`() {
        Faker().randomProvider.randomClassInstance<DoesNotWork>()
    }
}

Although I provide one specific way to reproduce this issue, anything that calls randomClassInstance can hit this bug. I believe this class of error could be handled for all cases by checking for constructorless cases at the top of randomClassInstance, but I haven't studied this question in detail.

If you're open to reviewing a pull request I'd be happy to submit one, but in either case thank you for your work on this project!