Nullable type error

val word: String = "abracadabra"
val freqMap: MutableMap<Char, Int> = HashMap<Char, Int>().withDefault { 0 } word.forEach { freqMap[it] += 1 }

results in this compile time error:
Operator call corresponds to a dot-qualified call 'freq[it].plusAssign(1)' which is not allowed on a nullable receiver 'freq[it]'.

What the hack!?
Why is freq[it] considered Int? when freqMap is of type MutableMap<Char, Int>

(Kotlin plugin 1.3.50-release-IJ2019.1.1)

See documentation:

Also there is a previous post that addresses this: Map.withDefault not defaulting?

Thank you, got it.