Map.computeIfAbsent requires not null result in Kotlin

Code snippet

val map = mutableMapOf<String, String>()
map.computeIfAbsent(key) { calcValue(it) }

with calcValue signature:

fun calcValue(key: String): String?

compiles with error:

Type mismatch: inferred type is String? but String was expected

But it is acceptable for mappingFunction to return null in Map.computeIfAbsent.

Is this a bug? Or am I doing something wrong?