<https://kotlinlang.slack.com/archives/general/p14...

# announcements

[ilya.gorbunov]

https://kotlinlang.slack.com/archives/general/p1447779888000839
iterator() is an extension function defined for read-only Map, thus it returns read-only iterator. Feel free to file a feature request to https://youtrack.jetbrains.com/issues/KT for an extension on MutableMap.
Meanwhile, you can call mutableMap.entries.iterator() as a workaround.