You configured MirrorMaker 2 but no data is replicating. Which is the most likely error in this config?

You configured MirrorMaker 2 but no data is replicating. Which is the most likely error in this config?

{
  "clusters": {
    "source": {"bootstrap.servers": "source.kafka:9092"},
    "target": {"bootstrap.servers": "target.kafka:9092"}
  },
  "mirrors": [{"source.cluster": "source", "target.cluster": "target", "topics": []}]
}