Spark Streaming from MQTT - IllegalArgumentExcepti

I'm trying to consume messages from RabbitMQ in Spark Streaming following this answer: https://stackoverflow.com/a/38172737/1344854. I'm failing with IllegalArgumentException and I don't know why.

6/09/05 13:23:22 ERROR ReceiverTracker: Deregistered receiver for stream 0: Error starting receiver 0 - java.lang.IllegalArgumentException: ssl://user:password@bunny.cloudamqp.com:8883/vhost
    at org.eclipse.paho.client.mqttv3.MqttConnectOptions.validateURI(MqttConnectOptions.java:458)
    at org.eclipse.paho.client.mqttv3.MqttAsyncClient.<init>(MqttAsyncClient.java:273)
    at org.eclipse.paho.client.mqttv3.MqttAsyncClient.<init>(MqttAsyncClient.java:167)
    at org.eclipse.paho.client.mqttv3.MqttClient.<init>(MqttClient.java:224)
    at org.apache.spark.streaming.mqtt.MQTTReceiver.onStart(MQTTInputDStream.scala:71)
    at org.apache.spark.streaming.receiver.ReceiverSupervisor.startReceiver(ReceiverSupervisor.scala:148)
    at org.apache.spark.streaming.receiver.ReceiverSupervisor.start(ReceiverSupervisor.scala:130)
    at org.apache.spark.streaming.scheduler.ReceiverTracker$ReceiverTrackerEndpoint$$anonfun$9.apply(ReceiverTracker.scala:575)
    at org.apache.spark.streaming.scheduler.ReceiverTracker$ReceiverTrackerEndpoint$$anonfun$9.apply(ReceiverTracker.scala:565)
    at org.apache.spark.SparkContext$$anonfun$37.apply(SparkContext.scala:1992)
    at org.apache.spark.SparkContext$$anonfun$37.apply(SparkContext.scala:1992)
    at org.apache.spark.scheduler.ResultTask.runTask(ResultTask.scala:66)
    at org.apache.spark.scheduler.Task.run(Task.scala:89)
    at org.apache.spark.executor.Executor$TaskRunner.run(Executor.scala:227)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
    at java.lang.Thread.run(Thread.java:745)

I tried passing various combinations of brokerUrl argument to the MQTTUtils.createStream constructor including tcp: tcp://user:password@bunny.cloudamqp.com:1883/vhost

Why is it failing to validate and how to fix it?