Error when trying to load JDBC sink connector

I am trying to stream data from a Kafka topic to a MySQL database unsuccessfully. Although the source connector works fine (i.e. streaming data from a MySQL database to kafka topic), sink connector fails to load.

Here is my sink-mysql.properties file:

name=sink-mysql
connector.class=io.confluent.connect.jdbc.JdbcSinkConnector
tasks.max=1
topics=test-mysql-jdbc-foobar
connection.url=jdbc:mysql://127.0.0.1:3306/demo?user=user1&password=user1pass
auto.create=true

When I try to execute

./bin/connect-standalone etc/schema-registry/connect-avro-standalone.properties etc/kafka-connect-jdbc/sink-mysql.properties

the following errors are reported:

[2018-02-01 16:17:43,019] ERROR WorkerSinkTask{id=sink-mysql-0} Task threw an uncaught and unrecoverable exception. Task is being killed and will not recover until manually restarted. (org.apache.kafka.connect.runtime.WorkerSinkTask:515)
org.apache.kafka.connect.errors.ConnectException: No fields found using key and value schemas for table: test-mysql-jdbc-foobar
    at io.confluent.connect.jdbc.sink.metadata.FieldsMetadata.extract(FieldsMetadata.java:127)
    at io.confluent.connect.jdbc.sink.metadata.FieldsMetadata.extract(FieldsMetadata.java:64)
    at io.confluent.connect.jdbc.sink.BufferedRecords.add(BufferedRecords.java:71)
    at io.confluent.connect.jdbc.sink.JdbcDbWriter.write(JdbcDbWriter.java:66)
    at io.confluent.connect.jdbc.sink.JdbcSinkTask.put(JdbcSinkTask.java:69)
    at org.apache.kafka.connect.runtime.WorkerSinkTask.deliverMessages(WorkerSinkTask.java:495)
    at org.apache.kafka.connect.runtime.WorkerSinkTask.poll(WorkerSinkTask.java:288)
    at org.apache.kafka.connect.runtime.WorkerSinkTask.iteration(WorkerSinkTask.java:198)
    at org.apache.kafka.connect.runtime.WorkerSinkTask.execute(WorkerSinkTask.java:166)
    at org.apache.kafka.connect.runtime.WorkerTask.doRun(WorkerTask.java:170)
    at org.apache.kafka.connect.runtime.WorkerTask.run(WorkerTask.java:214)
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)
[2018-02-01 16:17:43,020] ERROR WorkerSinkTask{id=sink-mysql-0} Task threw an uncaught and unrecoverable exception (org.apache.kafka.connect.runtime.WorkerTask:172)
org.apache.kafka.connect.errors.ConnectException: Exiting WorkerSinkTask due to unrecoverable exception.
    at org.apache.kafka.connect.runtime.WorkerSinkTask.deliverMessages(WorkerSinkTask.java:517)
    at org.apache.kafka.connect.runtime.WorkerSinkTask.poll(WorkerSinkTask.java:288)
    at org.apache.kafka.connect.runtime.WorkerSinkTask.iteration(WorkerSinkTask.java:198)
    at org.apache.kafka.connect.runtime.WorkerSinkTask.execute(WorkerSinkTask.java:166)
    at org.apache.kafka.connect.runtime.WorkerTask.doRun(WorkerTask.java:170)
    at org.apache.kafka.connect.runtime.WorkerTask.run(WorkerTask.java:214)
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)
Caused by: org.apache.kafka.connect.errors.ConnectException: No fields found using key and value schemas for table: test-mysql-jdbc-foobar
    at io.confluent.connect.jdbc.sink.metadata.FieldsMetadata.extract(FieldsMetadata.java:127)
    at io.confluent.connect.jdbc.sink.metadata.FieldsMetadata.extract(FieldsMetadata.java:64)
    at io.confluent.connect.jdbc.sink.BufferedRecords.add(BufferedRecords.java:71)
    at io.confluent.connect.jdbc.sink.JdbcDbWriter.write(JdbcDbWriter.java:66)
    at io.confluent.connect.jdbc.sink.JdbcSinkTask.put(JdbcSinkTask.java:69)
    at org.apache.kafka.connect.runtime.WorkerSinkTask.deliverMessages(WorkerSinkTask.java:495)
    ... 10 more
[2018-02-01 16:17:43,021] ERROR WorkerSinkTask{id=sink-mysql-0} Task is being killed and will not recover until manually restarted (org.apache.kafka.connect.runtime.WorkerTask:173)

Note that topic test-mysql-jdbc-foobar contains data streamed from MySQL to kafka however, I am not able to stream this data from MySQL back to kafka. The content of sink-mysql.properties looks identical to the one used in the official confluent's documentation but it doesn't seem to work. Also, mysql-connector is placed in the right directory (under share/java/kafka-connect-jdbc/).

EDIT

Here is the content of my worker config file:

bootstrap.servers=localhost:9092
internal.key.converter=org.apache.kafka.connect.json.JsonConverter
internal.value.converter=org.apache.kafka.connect.json.JsonConverter
internal.key.converter.schemas.enable=false
internal.value.converter.schemas.enable=false

key.converter=org.apache.kafka.connect.json.JsonConverter
value.converter=org.apache.kafka.connect.json.JsonConverter
key.converter.schemas.enable=false
value.converter.schemas.enable=false


# Local storage file for offset data
offset.storage.file.filename=/tmp/connect.offsets

plugin.path=share/java

To be able to use the JDBC Sink, your messages must have a schema. This can be by using Avro + Schema Registry, or JSON with schemas. In your worker config you've specified:

key.converter.schemas.enable=false
value.converter.schemas.enable=false

Which means that the JSON will not contain the schemas.

Here's an example of the JSON that Kafka Connect will produce (as a source) and expect (as a sink) if you enable schemas: https://gist.github.com/rmoff/2b922fd1f9baf3ba1d66b98e9dd7b364