pojo warning when using auto generated protobuf class

[Prashant Deva]

I am seeing this warning msg when trying to use a custom protobuf
de/serializer with kafka source with auto generated java protobuf class:

18:41:31.164 [main] INFO org.apache.flink.api.java.typeutils.TypeExtractor
- Class class com.xx.APITrace cannot be used as a POJO type because not all
fields are valid POJO fields, and must be processed as GenericType. Please
read the Flink documentation on "Data Types & Serialization" for details of
the effect on performance.

here is my serializer. What am i doing wrong?

class ApiTraceSchema: DeserializationSchema<Trace.APITrace>,
SerializationSchema<Trace.APITrace> {

    override fun getProducedType(): TypeInformation<Trace.APITrace> {
        return TypeInformation.of(Trace.APITrace::class.java)
    }

    override fun deserialize(message: ByteArray): Trace.APITrace {
        return Trace.APITrace.parseFrom(message)
    }

    override fun isEndOfStream(nextElement: Trace.APITrace): Boolean {
        return false
    }

    override fun serialize(element: Trace.APITrace): ByteArray {
        return element.toByteArray()
    }
}