java.lang.ClassCastException: [B cannot be cast to java.lang.String

We have a stored procedure called by Java using JPA:
StoredProcedureQuery storedProcedure = em.createStoredProcedureQuery(EXPORT_PROCEDURE_NAME);
storedProcedure.registerStoredProcedureParameter(OUTPUT_ATTRIBUT_NAME, String.class, ParameterMode.OUT);
storedProcedure.execute();
String jsonResult = (String) storedProcedure.getOutputParameterValue(OUTPUT_ATTRIBUT_NAME);

The last line raises an error:
java.lang.ClassCastException: [B cannot be cast to java.lang.String.
The output of our stored procedure is a string containing JSON formatted data looking like this: [

]
Here is the header of the "show create procedure" query:
STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION | CREATE DEFINER=`xxxx`@`localhost` PROCEDURE `EXPORT_TRANSFER_ALL`(OUT json_result TEXT)

This error seems similar is similar to the one reported here: https://mariadb.atlassian.net/browse/CONJ-28

It seems that we're receiving a byte array instead of string though we're not using a secured connexion to the database.