Lambda parameters "must be specified" for local lambda expression parameters

[BSteffaniak]

targetMap.forEach((pair) => {
  if (!map.containsKey(pair.key)) {
    map[pair.key] = pair.value
  }

  if (pair.value.class?.isOfType(HashMap.class)) {
    let HashMap<String, Object> hashmap = (HashMap)pair.value
    hashmap.forEach({ map[pair.key].add(_) })
  } else {
    map[pair.key] = pair.value
  }
})

produces "Error: Types must be specified for local lambda expression parameters"