oh ok.

You should do that instead then :
```scala

case class PrefixedKey(prefix: String, key: K)(implicit redisKey: ByteStringSerializer[K])

object PrefixedKey {
  implicit val serializer = new ByteStringSerializer[PrefixedKey] {
    def serialize(data: PrefixedKey): ByteString = {
      ByteString(data.prefix + data.redisKey.serialize(data.key))
    }
  }
}

val prefixedKey = PrefixedKey("prefix", binaryKey)

redis.send(Exists(prefixedKey))

```