Is it impossible to select multiple booleans into variables using Informix?

I am facing a strange problem when selecting multiple boolean values into variables using Informix SPL.

It works fine to select a boolean value into one variable. It also works fine to select multiple integer values into variables.

What does not work is listed below:

CREATE PROCEDURE test()
    DEFINE b1 BOOLEAN;
    DEFINE b2 BOOLEAN;
    DEFINE i1 INTEGER;
    DEFINE i2 INTEGER;

    SELECT FIRST 1 't' INTO b1 FROM systables; -- OK
    SELECT FIRST 1 'f' INTO b2 FROM systables; -- OK

    SELECT FIRST 1 1, 2 INTO i1, i2 FROM systables; -- OK
    SELECT FIRST 1 't', 't' INTO b1, b2 FROM systables; -- FAILS!

END PROCEDURE;

EXECUTE PROCEDURE test();

The error message is:

SQL Error (-9635): An attempt has failed to convert an opaque type into another type without a cast function.

How can I avoid this error?

This appears on IBM Informix Dynamic Server Version 12.10.FC10WE; the OS is Linux.