ZEOSLib: Error Message from TZReadOnlyQuery

Question

I have been using ZeosLib Components with Delphi and Lazarus for the past 10 years or so. I'm mostly using it to access MySQL Databases. Now I have run into a strange Problem with TZReadOnlyQuery.

In my MariaDB database I have a pretty complex stored procedure that takes some input parameters, one inout param and one out param. Because of the error I get, when I try to call it with TZReadOnlyQuery, I have made a very simple procedure to test it.

This is my test procedure:

CREATE PROCEDURE MyProc(INOUT a VARCHAR(255), OUT r VARCHAR(255))
BEGIN
  set a = 'inparam changed';
  set r = 'outparam set';
END

I tried to call this procedure from different MySQL query/managment tools with this statement:

set @x = 'inputparam';
call MyProc(@x, @y);
select @x, @y;

I get the expected answer: a dataset with one record and two fields like this:

But when I add the same query statement to a TZReadOnlyQuery and try to open it I get an error message:

Any ideas why this happens and how to work around it?

Thanks!