Transform values of a Map in groovy

Lets say I have a map Map<String, List<Integer>>.

I want to transform this map into Map<String, Map<Integer, Object>> by applying convert() method for each pair of key and element of nested list.

Object convert(String key, Integer value)

How can I achieve that?

I tried something like this:

map.collect { key, list ->
    key: list.collectEntries {
        [(element): convert(key, element)]
    }
}

but I'm getting ClassCastException: ArrayList cannot be cast to Map.