             return nil
         }
 
-        return streamIDs.map({ (streamID) -> AudioStream in
-            AudioStream.lookup(by: streamID)!
-        })
+        return streamIDs.map { AudioStream.lookup(by: $0) }.flatMap { $0 }
     }
 
 

        var instance = AudioObjectPool.instancePool.object(forKey: NSNumber(value: UInt(id))) as? AudioDevice

        if instance == nil {
            instance = AudioDevice(deviceID: id)
        }

        return instance

    public func clockSourceNames(channel: UInt32, direction: Direction) -> [String]? {

        if let clockSourceIDs = clockSourceIDs(channel: channel, direction: direction) {
            return clockSourceIDs.map {
                // We expect clockSourceNameForClockSourceID to never fail in this case, 
                // but in the unlikely case it does, we provide a default value.
                clockSourceName(clockSourceID: $0) ?? "Clock source \(clockSourceID)"
            }
        }

