Invalid byte in codeset conversion input

When I insert the string (which is converted to UTF8 from Big5) into the database (DB_LOCALE on the server side =en_us.UTF8), it showed the error message "Invalid byte in codeset conversion input". In fact, there is one character not in unicode in the string. The DB_LOCALE and CLIENT_LOCALE on the client side both are set to "en_us.UTF8". It is supposed not to do the code conversion when DB_LOCALE = CLIENT_LOCALE according to the Informix Manual. Why the conversion happens or the non-unicode character is out of the scope the verion of Informix Database or other support?