Informix JDBC PreparedStatement UPDATE Multiset

Question

I want to change MULTISET column of the table, and my code looks something like this:

PreparedStatement pstm = preparedStatement("UPDATE table SET mc = MULTISET{?, ?} WHERE ...");
pstm.setString(1, "...");
pstm.setString(2, "...");
pstm.execute();

And I get error: "Illegal attempt to convert a collection type into another type.".

What is wrong with this? When I put real values instead of '?' everything works just fine.

Btw... mc is defined like MULTISET(CHAR(20) NOT NULL)

Thank you...