does this prevent mutable.HashSet from having a negative size?

I have a mutable set
private val connectedUserIds = collection.mutable.Set.empty[String]
and according to monitoring the size of that set is negative
mon.sets.users(connectedUserIds.size)
Negative histogram value: socket.remote.sets.users=-194

Now I'm not a CS expert, but I'm quite sure sets are not supposed to have negative sizes

modules/socket/src/main/RemoteSocket.scala

@@ -35,7 +35,9 @@ final class RemoteSocket(
  case In.ConnectUser(userId) =>
  bus.publish(lila.hub.actorApi.socket.remote.ConnectUser(userId), 'userActive)
  connectedUserIds += userId
- case In.DisconnectUsers(userIds) => connectedUserIds --= userIds
+ case In.DisconnectUsers(userIds) => userIds foreach { userId =>
+ connectedUserIds -= userId
+ }
  case In.Watch(gameId) => watchedGameIds += gameId
  case In.Unwatch(gameId) => watchedGameIds -= gameId
  case In.NotifiedBatch(userIds) => notificationActor ! lila.hub.actorApi.notify.NotifiedBatch(userIds)