Hi all. Got a strange error related to typealias &...

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[dimsuz]

Hi all. Got a strange error related to typealias && extension functions:

data class Custom(val i: Int)

typealias MyData = Map<Custom, List<Int>>

fun MyData.isEmpty(key: Custom) = this.get(key)?.isEmpty()

get invocation is hightlighted and the error is:

"Type inference failed. The value of type paramter K should be mentioned in input types. Try to specify it explicitly"

Any hint of how to do that and why is it needed?
If I replace Custom with String there, no error is produced.