Null value in the base class when using inheritance

I have the following Kotlin code using with spring, I get a NPE.

When the defaultExt get used in the save method, it’s a null, I don’t understand, I pass it as "png".

class CoverRepository(
    @Value("\${photo.location}") root: String,
) : PressBaseRepository(root, "png", "cover") {

abstract class PressBaseRepository(
    root: String,
    private val defaultExt: String,
    private val type: String
) : DiskRepository(root) {

    fun save(id: Long, lang: Locale, cover: MultipartFile): String {
        val ext = parseExt(cover, defaultExt)
        val filename = "press_%s_%d_%s.%s".format(type, id, lang.toLanguageTag(), ext)
        saveToRoot(filename, cover)
        return filename


java.lang.NullPointerException: Parameter specified as non-null is null: method com.pagepilot.v2.repository.core.FilenameKt.parseExt, parameter default