thank you guys for explaining all that 🙂 managed to put it all together and get it working with mapValues

errors.mapValues { (_, v) ->
                     when (v) {
                        is String -> listOf(v)
                        is List<*> -> v as List<String>
                        else -> listOf(v.toString())
                    }
                }

Won't the last case account for the 2nd one as well? Basically doing a

.toString()

on a list of a different type should still get you a list of strings after

listOf()

maybe pragmatically, but the type system doesn’t know that this map is really

Map<String, String|List<String>>

ah right, makes sense